Hmm... i don't know, but that could be the reason, or one of the reasons.
In all fairness though, Ngannou was on a six fight finish streak inside the UFC (10 finishes in a row overall) himself, but i agree that Stipe was (and still is) severely underrated by many.
I think the fact that Ngannou looks more like a cartoon character than Stipe does has shaped this narrative as well.
Fans of combat sports should know better, but so many people still fall for athletic/muscular looking fighters, it's crazy... (both Stipe and Jan have a super wide upper body, possibly making them look sluggish to some people too).
Last but not least, people always expect more outspoken guys to be the better athlete's if that makes sense - Stipe and Jan are both very usual (in a good way imo) and unspectacular guys outside the cage.