Unless a CA-licensed attorney knowledgeable in California gun laws and ATF regulations specifically told you that a particular CA law applying to "semiautomatic firearms" does not apply to your AR-15 lower in isolation, do not listen to random internet comments about it especially when they begin with "I believe." Regardless of where you fall on the issue politically or ideologically, it's an objective fact that California's government is hostile toward its citizens possessing and bearing firearms, and being fuzzy on what is or is not a felony is a risky proposition.
If you're wrong, best case scenario you will lose all your guns, worst case you will end up in federal prison.