I think generally UBTI refers to income that is actually unrelated to the good the nonprofit purports to do.
For example, Princeton a nonprofit can totally charge tuition and use it as a base of funding, and not pay taxes on that. They can even sell their students food in cafeteria and not pay income tax. [1] But they can't start a food franchise and not pay tax.
This software seems clearly within their registration statement. Now if they started selling software consulting as their major source of funding...
In any case, I agree checking with your accountant is the best.