KAPA is indeed 5900 MSL, and those heights are also all MSL. Looking at the FAA's charts, https://aeronav.faa.gov/d-tpp/2104/05715R17L.PDF https://aeronav.faa.

KAPA is indeed 5900 MSL, and those heights are also all MSL. Looking at the FAA's charts, https://aeronav.faa.gov/d-tpp/2104/05715R17L.PDF https://aeronav.faa.gov/d-tpp/2104/05715R17L.PDF, this combination of heights and distances isn't totally crazy -- I state without proof (and wait to be corrected!) that the approach is modified because it is a relatively high altitude airport and there may be quite steeply changing terrain underneath. I've never flown there (not even on X-plane) and I'm a low-hours UK person. I haven't done the trig and meterological lookup to work out what their AGL altitude was on that day, but at the very least it's not crazy-wrong from the published chart...