Do you need to own a platform to have a de facto grip on game distribution?

It helps immensely to own the platform you're also distributing software for if you're planning to enforce platform-specific restrictions, such as restricting which storefronts can even operate on your platform. Yknow, like Apple ~~does~~ did. But that aside, Valve does not have a de facto grip on game distribution because multiple platforms exist where Steam doesn't even distribute games (Microsoft Store, PlayStation Store, Nintendo eShop, etc.), and the only gaming platform that Steam does occupy has multiple competitors (Epic, Uplay, EA Play, GoG, itch.io, etc.).

I like Steam as much as the next guy, but it’s totally douchey the way nerds fall all over themselves to shit on Apple, but not Valve for charging the same thing

There's reasons to shit on both of them, but Valve taking an initial 30% cut of games sold on their own platform makes sense. They offer way more services than the competition, and frankly developers don't have to use Steam. They can use any of the other aforementioned platforms to distribute their games, or just roll their own platform if they're daring and patient.

But, I guess not “owning” a platform makes you immune from criticism.

No idea how you came to this conclusion. Both companies have legit criticisms made against them that have pretty much nothing to do with the case discussed in the article. Apple does flat out anti-consumer, and sometimes anti-developer shit all the time, Valve's work culture isn't near as diverse as it should be in the 21st century, and they don't seem to do any sort of audits of new games they distribute, they also don't seem to care about abandoned titles people have already paid for, etc.

Given that “owning” the platform is the problem, then I’m hoping to see an equal amount of rage at Sony, Nintendo, and Microsoft for their online stores that charge 30% to distribute games.

That's... not the problem though. Did you read the article? This is in relation to a class action lawsuit made by some disgruntled developers being put off by Valve's 30% cut on a platform where they have the option to use some other service lol. Sony, Microsoft, and Nintendo are the only official distributors of digital games on their respective platforms.