Huh. I always thought monks were a little underappreciated as damage dealers, especially if you're not using GWM/SS. Nice confirmation.
I'm not following the math provided, but generally monks are a bit underpowered.
For example, let's say I have standard array of stats and two 8th level PCs: Battlemaster Fighter with GWM and a 20 strength and a dexterity 20 monk. They are each going to have a long rest, fight 3 encounters, take a short rest and fight 3 more encounters before their next long rest. Each combat is 4 rounds.
The fighter gets multi-attack, a 2d6 weapon, action surge twice, 10 d8 superiority dice (assume all are ripostes or otherwise grant an extra attack and damage). That is attacking at +3 for 2d6(average 8.33 due to GWF)+15 each time, with 39 (one bonus attack from GWM on average in the 28 attacks on the fighter's turn) attacks and an additional 10d8 superiority damage. Versus AC 15, that is about 310 damage.
The monk gets multi-attack, a d6 weapon, and 16 flurries for 2 attacks each. That is +8 for d6+5 for 40 attacks. That is about 245 damage.
Now, if you can add Hex in there that becomes +8 for 2d6+5 for 40 attacks it is about 350 damage. However, getting Hex for attacks on both sides of the short rest is hard without spending a level on another class. Further, once you hit 11th level and the fighter advances up to 3 attacks per round, even the Hex Monk begins to fall behind. Plus, few monks use all Ki on extra attacks - not when they might be using their path abilities.
However, I did play in a one shot where the DM provided me with a 9th level human monk with a flame tongue shortsword and a human variant feat of warlock magic initiate. He was.... effective with 4d6+5 attacks.