None of this prohibits users from modifying their bootloader, kernel, or OS image; but any such modification would invalidate the secureboot signature and thus break attestation until the user registered their own signatures with the EU.
The EU currently only transacts with Google in this regard because, as far as I know, they are the only Android OS publisher (and perhaps the only Linux publisher?) that bothered to implement hardware-to-app attestation chaining live in production end-user devices in the decades since Secure Boot came onto the scene. All it takes to change that is an entity who has sufficient validity to convince them that outsourcing permitted-signature verification to Google is unethical, which it is.
It’s a safe bet that Steam Linux was already working on this in order to attest that the runtime environment is unmodified for VAC and other multiplayer-cheating prevention systems in games — and so once they publish all that, I expect we’ll find that they’ve petitioned their attested OS signature chain to the EU as satisfying age requirements for mature gaming.
The vendor lock-in here is that Apple and Google and, eventually, Valve, are both willing to put the weight of their business behind their claims to the EU that they do their best to protect the security of their environment from cheaters, with respect to the components required by the EU age verification app. The loophole one could drive a truck through that the EU has left open to break that lock-in in the future? Anyone can petition the EU to accept attestations from their own boot-kernel-OS chain signatures so long as they’re willing to accept the legal risks visited upon them if found to have knowingly permitted exploitation for age check bypasses, or neglected to respond in a timely and prudent manner when notified of such exploitability by researchers — and if the EU rejects their petition improperly, they’ll have to answer for that to their citizens.