How so? Given that the license says you can't "use" it for commercial purposes, as opposed to just that you can't modify or distribute it for commercial purposes, what else would that mean?

It means you can't sell it/make a profit from it. There's a legal definition of "commercial purposes":

https://www.law.cornell.edu/definitions/uscode.php?width=840...

That's a legal definition of "used for commercial purposes" for vehicles: https://www.law.cornell.edu/uscode/text/18/31, https://www.law.cornell.edu/uscode/text/18/part-I/chapter-2

Isn't performing paid work activities with a keyboard application profiting from using the keyboard?

Are you selling and profiting from the keyboard software _as a product_? No, you're not.

Say I take the keyboard software/app _itself_, rebadge it as ohmykeyboard, then sell it for profit. I would be violating the license in that case.

Just typing with it and using it on a regular basis doesn't violate that part of the license.