They’re forgetting about federal properties. BLM, parks, military bases, etc are of significant size & value, and may not be abandoned by the legal owner.
Remember: the Civil War started when Fort Sumter was attacked for being occupied by a now foreign government, and the USA doesn’t end until the attacker unconditionally surrenders. CA isn’t ready for a full on war with the USA.
Unlike the Civil War, we have airborne terminators to wipe out secessionist government officials and bypass any forts or military bases the secessionist military has.