still became a general in the confederate army due to his loyalty and his duty towards the state of California

In the most literal sense, you are very wrong, as he was a Virginian, not a Californian, of course... More broadly though the separation of Lee from secession and slavery by ascribing his motives solely to high minded attachment to Virginia is a common narrative, but it is one grounded in a mythos of the Civil War that was heavily influenced by the Lost Cause, and which, sometimes implicitly and sometimes explicitly, downplays of entirely does away with aspects of Lee, the man, which did, in fact, support secession and slavery.

I've written more on this here, as did /u/secessionisillegal in the same thread, and I would also point to /u/sowser and his own writings on Lee here.