OP asked for FOSS, and if this keyboard had met OPs other criteria, it would have failed the FOSS check (it’s a source-available license). It’s also a roll-your-own license and a very very short one at that. It’s missing a lot of key protections for both the company and the consumer.
I’m pretty steadfast on using GPL software wherever i can, especially for something as mission-critical as a keyboard. Non-gpl projects have a tendency to get bought up and relicensed or corrupted in some other way over time (sometimes a very long time, but time nevertheless). I’ll make exceptions for things that are less critical, like games, but core system must be GPL or offer equivalent protections for the end user.
Source-available is still good for auditability though, making it more secure in the short-term.