Yes but its a double whammy with the underslot approach.
When you get to your pick. Whomever is available IS available. Certainly it might not be your targeted first choice but its your pick now and you can have whomever you want that is left on the board. Going underslot because you want to target guys later in the draft just double down on not getting who you want. If they end up not being available (like apparently last year) then you end up not getting who you wanted with your first pick AND the subsequent picks.
I imagine that teams going underslot in the first are comfortable with several different permutation of the rest of the draft unfolding. Certainly one or two prime targets they hope will be available at a later pick, but lots of routes they'll be happy with. I'd agree that a gameplan that revolves heavily around one specific guy being available later on is a flawed plan.
I also do think that it's probably somewhat common that teams genuinely feel the guy they take is an equal or better prospect than everyone else available, he just happens to also have lower demands and they can get some extra bullets to improve their draft later on as a bonus and not even really as an alternative. I imagine that there are tons of disagreements on various boards throughout the league. Was Henry Davis chosen largely as an underslot candidate or did the Pirates just like him the best of everybody?