"Underbiked" does not mean that you CAN'T ride something. Or that you won't enjoy it. It just means that bigger bike would make the task easier and you would likely be faster.
Of course you can ride Slickrock and many other famous Moab trails on a rigid bike with cantis.....People have been riding those trails since that was all there was (or since the suspension forks were not any better than rigid). And of course they enjoyed it. Well, some of them, at least. And maybe some folks still would enjoy it.
And since there were no other options, the term "underbiked" did not really mean anything back then. But now there are other options, so the term could apply now.
FWIW, I don't think Slickrock Trail itself requires much in the way of suspension. It has been 15 years since I rode it, but as I recall, it was relatively smooth. It was just ridiculously steep in places. Like, I was hitting my knee on the topcap of my fork climbing some of it. Stuff like Porcupine rim and other sections of the Whole Enchilada... my body hurts thinking of doing that ride on a rigid bike with cantis. I'm too old for that.
