I see no problem if Illinois has made this requirement for other Presidential candidates. If not, Illinois can go pound sand.
We have not seen the law as written.
However the Constitution takes a dim view of laws that are written to apply only to one person.
I suspect that to make the law constitutional that the law would need to require each and every candidate running to divulge his/her tax returns.
Since a problem with a tax return would or could prevent a candidate who won the primary from taking office, it would be necessary to have all candidates in both primaries divulge their tax information.
See where this would go?
There are obviously primary candidates who would never want their information disclosed. And most of them are dims.
As usual, dims are just that; dim.