Unless the bill has be changed since last time I read it, it's severely flawed(As to its apparent intent). It's designed to prevent usage of parts to convert from fixed magazine to detachable, not the other way around. Particularly since it relies on current definitions already in CA law. By current definitions, an AR, AK or other rifle with "evil" features with a bullet button is completely legal(minus those listed by name in the two separate AWB bills).
The bullet button or similar device by current definitions makes it a fixed magazine. The only things this would prevent would be things like the mag magnet(already illegal anyway as the OP noted), and parts used to enable SKSs and other fixed magazine rifles to accept magazines(It should be noted SKSs with detachable magazines are already illegal, I just used it as an example as it was the first fixed magazine rifle that people convert to accept detachable magazines that came to mind).
Granted this bill is another arbitrary restriction that will hopefully not pass, but it is an amazing example of legislation without understanding.
Although on the non-serious side, if the bill were extremely broadly interpreted
screw drivers and hex wrenches might become illegal, as they are "parts" used to convert firearms from fixed to detachable magazine.