[antonvs]

I was wrong. Apparently it is slang, in some places at least:

> The word penultimate as a slang word seems to have worked its way into common parlance thanks to the slang use of the word ultimate. As a slang term, ultimate means cool rather than last. So the hipster logic may have concluded that if ultimate means cool, then penultimate must mean super cool.

https://minnesota.publicradio.org/radio/podcasts/grammar_gra...

I assumed the parent was making a joke that something like this would be the case. Turns out it really is. "Never underestimate the stupidity of..."