Actually, technically, yes.
They could be counted, and congressional seats assigned, if they are here during the census and they fill the census papers.
I doubt, however, that too many real tourists bother!
And most of them probably do not think they are eligible.
They may not need to fill out the survey: the Feds are placing illegals where they want them and recording their locations, so as far as they are concerned, there's an accounting. You and I might not think that's a proper count, but we're also not the ones doing the tabulation.