I know all that.
When I say uber-boss, Im saying it within the context of the 6 or so encounter adventuring day (2 short rests) which is also the context I use to gauge Monks power.
For example 8th or so level PCs (at an eyeball) dealing with a nest of Hobgoblins with an alliance with a Blue dragon:
E1: 2 Trolls (random encounter on the way there)
E2: 8 x Hobgoblins, 2 x Hobgoblin Captains (guard the entrance)
[short rest]
E3: 2 x Half-[Blue]-Dragon Veterans (add martial advantage as they're Hobgoblin/ Dragons)
E4: Mage [Hobgoblin skin], 2 x Elite Hobgoblins (Re-skinned Knights with martial advantage) 4 x Hobgoblins
[short rest]
E5: Hobgoblin Warlord, Hobgoblin Devastator and 6 Hobgoblins
E6: Young Blue Dragon (tack on 3 legendary actions of search, tail slap and wing buffet, and add 3 legendary resistances, increase HP by 50 percent, increase CR to 11)
Plus a few traps and so forth.
I can see a Monk doing just fine in that adventuring day, stun locking a fair few boss things as he goes and more than pulling his weight.