Unfortunately Itch does not allow creators to list their creations under any licenses except for the Creative Commons licenses. CC-BY-SA4 seemed like the closest match to the SIL Open Font License for metadata purposes.
The only other choices are “All Rights Reserved” and “No License”.
I agree, it is disappointing. But just as user I don't like SIL Open Font for requiring distribution with a program (Yeah simple "hello world" is enough but it is...)