An article on Medium puts forth the proposition that FOSS is really just capitalism by another name. I agree with some parts, but mostly disagree with this idea.

FOSS is neither pro nor anti-capitalism. Although it can be used to help marginalised people there's nothing in the four freedoms which says that's its primary goal. Really it's just a development methodology which encourages sharing and collaboration rather than competition and secrecy. The sharealike nature of copyleft licenses do bias this type of development away from the exclusionary nature of market competition within a capitalist economy, but there are still plenty of very capitalistic companies using software with copyleft licenses and even sometimes developing new software under those licenses.

work is not acknowledged under capitalism unless it is measurably productive and benefiting someone who is already wealthy

This I agree with. At any point in time there's plenty of work to be done and things to be fixed, but unless doing those things personally benefits some rich person then typically there are zero funds available. Notice the badly maintained roads and railway infrastructure or city parks, for example.

But the rest is mostly wrong in my estimation. Even the first line:

Free and Open Source Software (FOSS) positions itself as being apart from capitalism

isn't really true. The original GNU manifesto mentions capitalism, but implies that it's good when the market competition is fair and that being non-proprietary helps to ensure fairness.

FOSS is exactly the same as capitalism in this way [that non-coders can't fork and continue a project], but with no greater governing body to create and enforce anti-discrimination laws. It is therefore safer for marginalised people to use centralised software under large companies that are accountable to the law.

Marginalised people are often marginalised precisely because the law doesn't work in their favor, or that laws are selectively applied and that if you're marginalized then you don't get any chance of legal representation. As a practical example, in recent years access to legal aid has been greatly reduced in the UK and this means that even if the law is on their side many people have no access to it.

Also the centralised systems have usually been the worst offenders when it comes to the rights of marginalised people. Over the last five years a lot of immigration into the fediverse has been precisely due to people being blacklisted by centralised systems for purely arbitrary or discriminatory reasons.

In companies the law ensures that marginalised people are treated appropriately, and progress is slow but we’re getting there. In FOSS the only tool we have is user pressure, and it’s not working. All the power is with the developers, who have the time and/or money to be able to code because they’re in a privileged group. In FOSS as in capitalism, power begets power, and those at the top don’t share.

This is a characterisation of companies which I don't recognise. Unless you're working within a cooperative or are self-employed, companies are structured in a feudal manner. The law provides very little effective protection of employees, again also because few people have the money to be able to make use of legal services. If the boss breaks employment law usually there are no repercussions. You only stand a chance if you have the equivalent of "user pressure", i.e. something like a union or trade organisation independent from the company. In the end the law is not a substitute for real solidarity.

Free Software developers have the power to make or fix software, but usually they don't have much other power or privileged access to resources. Like raising children and other kinds of domestic work, FOSS is often not recognised as being "work", is usually unpaid and mostly doesn't appear in the GDP figures. It might seem that FOSS developers are incredibly privileged if you take the employees of Google or Facebook as an example, but those people are really just a tiny number compared to the set of all active FOSS developers, and they're not even the most productive ones.

The complaint that FOSS developers don't share power is really a conflation between two different things. FOSS is about sharing software. It's not really about sharing software making skills and it doesn't imply any particular governance model. Individual engineers aren't obligated to design their software by vote, although in some projects that may happen. There certainly are problems with the "benevolent dictator for life" (BDFL) governance model, particularly when projects become large like the Linux kernel. Mastodon currently is also suffering from the limitations of BDFL.

The problem with BDFL is that nobody is really all that smart and that no matter how empathetic you are it's always difficult to know what other people's software requirements are unless you have those requirements yourself and in the same sort of context. Trying to design things in the high-minded belief that you know what's best for other people is how a lot of activism ends up being ineffective. It's why having a diverse software development team and some non-BDFL governance model is useful.

One important thing to keep in mind though is that most FOSS projects have only one or two developers and so never encounter the problem of governance at scale. Also production really is key. You can debate things long and hard, but in the end its action which matters.