It’s posturing. If push came to shove they’d gladly allow them to be sent back.Not really all that hard, funding can be cut off easily. Congress controls the purse strings not courts. When you do things "properly" they will lose and I seriously doubt they will have the will to fight. Appropriated funding matters but performance matters as well. (Many factors but it can be done.)
Making California an open air cage for illegals while their tax paying citizens come to red states would be a sad affair, but better than enduring what’s happening now.