That one is murky.
I would argue it falls more on a case like the Linux distributions one and the guy in South America is responsible.
Google, in my opinion, can be held accountable for Android because they deeply control the ecosystem (App Store, APIs, Services) and de-facto prevents significant modifications, specially when they deal with Android security framework & mechanisms (if you modify this stuff too deeply, the Apps could break).
But if a distribution cuts ties with that, then it's the author or the entity behind it who is responsible, and if it's deemed illegal, downloading his ROM should be blocked in the US.
In truth, if I were a defender of this law (which I'm not), I would not worry too much about it. This text is here to force mainstream OS vendors to provide an API for age verification. The micro-subset of people flashing custom roms onto their phone or recompiling a piece of OSS software with some flag disabled is in practice so small that it's not really an issue.
While I do agree that this law could be better written, properly categorizing in it the cases of MS, Apple, Google and maybe entities like Linux Distribution honestly is kind of a nightmare.