Asking a question about citizenship is not excluding anyone from the census
There argument is, that the citizenship question will scare away illegals from participating in the census and thus they won’t get counted, and thus California will lose representation. Since illegals have already disregarded US law, it’s not a stretch to think they’d disregard US law requiring cooperation with the census.
Since the census is used to apportion representation in Congress, and since only citizens are legally authorized to vote for those representatives, being able to count actual citizens, separate from illegals, sounds like a good idea to me.