Oh it’s dirty for sure, and not a patina - but it’s interesting that after just 1 days use, that it looks like that. After a clean with a very lightly dampened cloth, it looks like this (Pics taken on a 10.5” iPad Pro, apologies they’re not as sharp as you might expect from a 12 Pro Max)
I learned that when cleaning the case with a wet cloth it's not only the dirt that comes off, also a fair bit of the coloring of the case