The OP didn't say there was any chance of him running the cyclist over. He was going the opposite way and looked behind.
The cyclist was very likely to have pedal reflectors - people who use clipless tend to have lights - and quite likely to have a red reflector. He might also have had reflective piping on dark clothing. None of these would have shown up in a rear view, but would make the cyclist perfectly visible to a motorist driving up behind him.
Stupid, yes, but not suicidal.
pure speculation.
Um, yes.... You didn't see the words "very likely", "quite likely" and "might" then?
My point is, on a balance of probability, the cyclist was more likely to have reflectors than not have reflectors. The OP didn't know how visible the cyclist was to a driver approaching from behind but assumed he would not have been visible.