The goal here is two-fold: One cause general terror in blue-states and 2 reduce blue state population in anyway to help increase GOP representation in the House.
Districting is not based how many citizens live in an area, but how many residents. Chipping away at blue state population in anyway helps red states get more seats in the house. And, I think they believe California is 99.99% undocumented immigrant, so they think it'll hurt the blue states/cities a lot.