Depressurization would have frosted the windows. Hypoxia is the human loss of oxygen. It could have many causes.
There was a recent General Aviation crash because a supplier of oxygen bottles filled some with pressurized atmospheric air instead of pressurized O2.
The pilot suffered hypoxia, passed out and was subsequently killed.
Another case was a faulty O2 line that leaked. The copilot suffered hypoxia and fell into unconsciousness but the pilot survived and landed.
None of these hypoxia cases frost windows.
" McSpadden said that based on known information, two factors likely could have led to the crash, hypoxia or rapid decompression."
What caused his "Hypoxia"?
N611VG Was NOT a loss of Pressurization. Part I
N611VG Was NOT a loss of Pressurization. Part II
AOPA; and Richard McSpadden; should check this out also.