How does precedent about how commercial is interpeted here apply? If I use this to fire off a work email, is that commercial? Or does this only apply if I'm selling a product built around this?
I'm the person responsible for this project and this is absolutely not correct. The clause was not written to restrict this kind of use of the keyboard, and it would be ridiculous if we ever pursued anyone over this. You can type anything you want with our keyboard. You can type out a million-dollar app that competes with us. It's not like we would have any way of knowing, because the keyboard doesn't connect to the internet.
That clause is more intended for a situation where, for example, a phone manufacturer wants to include our app as a default option. That is the kind of commercial use we would prefer to negotiate and sign a special agreement for.
While it was not your intention for the license to be interpreted in that way, as written, reading the text, it does seem like the most obvious interpretation. There are also other source-available licenses where it is the intended interpretation: while niche, I'm reminded of the license on nupack.org. It seems like clarifying this in the license text would be a good idea.
You have discovered why licenses are hard! The text of this license conflicts with your intent.
Wrote this comment with FUTO and it's pretty nice; it's certainly the first one I'm considering using over Gboard (because frankly the feature set of many FOSS keyboards is quite bad). While not FOSS, FUTO is a nice step up in terms of philosophy and actual license from most proprietary keyboards, with the exception of the commercial use restriction which really made me think twice about it.
I trust you that it's not what you meant, and that you wouldn't sue over it, but not that any potential future copyright holder wouldn't (see, e.g., https://gavinhoward.com/2022/01/the-law-of-strict-licenses/). Could you make it less ambiguous by chasing the license to only restrict commercial distribution rather than commercial use?
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?