Interesting! It was my understanding that TSA Precheck was lumped in with Global Entry, NEXUS, and SENTRI, all of which make up DHS'/CBP's Trusted Traveler Programs [1] [2]. I have Global Entry, so have not used the TSA Precheck enrollment flow (I steer those who ask to Global Entry, as it's only $15 more over 5 years vs Precheck and gives you everything TSA Precheck does but also CBP expedited clearance benefits at the US border). I'll have to do more research, thanks for pointing this out.
Interestingly enough, the TSA Jobs portal does use Login.gov. [3]
[1] https://login.gov/help/specific-agencies/trusted-traveler-pr...