Actually the bill seems to address those points adaquately.
The requirement to collect the user's age bracket at account creation, so no accounts means no account creation, so no requirement to collect the flag. If FreeDOS or your new OS has no accounts but does have an app store, then I think you could collect it on first use without trouble.
If you were developing an FOSS OS I think you could make a good arhument that you were not the “Operating system provider” as you don't control the opperating system on the end user's computer, and had no control over them installing it. That would leave the Californian end user to handle it themselves.
If you read the bill itself it actually looks like it's designed to minimise the risks associated with age gating, and provides various get out clauses that mean that app designers can avoid collecting intrusive information and must instead rely on what they're told by this flag. Without something like this, developers instead need to rely on third parties like Persona that use ID dicuments and videos to validate users, and then, inevitably, get hacked.
As I said before, all if this is based on the assumption that age gating will happen, and that is looking like a certainty. Given that, I'd much rather have laws like this than ones requiring privacy destroying and intrusive checks.