This is the best use of the limited line item veto I have ever seen by any Wisconsin governor.
A little bit of history, the original line item veto allowed the governor to veto any letter or number combination, effectively writing legislation letter by letter (dubbed the Vanna White veto). Limits were established that only phrases could be struck out, and look what Tony Evers did here.
For anyone that's interested, more on Wisconsin's Line Item Veto here.
So not entirely correct, the reason this is legal is because the amendment was only for letters (not numbers) and in fact the legal committee agreed that it allowed the creation of new numbers.