You're right, scratch that.
I've just never downloaded apks directly from Github and got confused by that issue about naming conflicts with the original OpenBoard. If you have installed that one, you need make your own debug build to run them in parallel.
Unfortunately, it's an abandoned project that hasn't gotten updates for 2 years.
There's an active fork but no release yet, you can only install by building it yourself: https://github.com/Helium314/HeliBoard