Nice analysis. My first thought was he is under powered (which he is in a way), but I did notice the 120' teleport legendary action.
Yeah, I did exactly the same. Took a look at his base numbers and thought: weaker than a pit fiend! But raw AC/HP/DPS only matters in white-room analysis. A mix of DC 21 stuns, 120' range teleports and 30' forced movement attacks on the player's turns is pretty scary.
He's a melee character's worst nightmare, and poses a very interesting tactical challenge on the battle map. If the party stays close together, they're grouped for a
fear breath or
stinking cloud - which are very nasty, as they knock the target out for at least one round. If they split up, then Moloch can cut them to pieces individually. Picture four PCs, all more than 30' from each other. Moloch teleports 30' away from the most attractive target, pulls him in with the whip (both legendary actions!), then pounds him with a full attack. The other characters are all a minimum 60' away; some will be further. Even if one of them manages to close and attack in that same round, Moloch uses another legendary teleport after the first responder to get another 120' distance. Even some spellcasters might not have the range. Once all sources of radiant damage are down, it becomes a massacre; good times.
I remember the only time my players encountered a storm giant quintessent (Volo's Guide to Monsters). They were pretty amused in the first round. CR 16 creature with AC 12? Sure, it
always hits with wind javelins... but for only 38 damage per round. What a joke! Then, as a legendary action after the first player's turn, it used One with the Storm and dispersed (untargetable). It reformed at the start of its round, 600' away from the party. That's the maximum range of its wind javelins. It threw two more, for another 38 damage. The players' faces went white - none of them had a longbow, which is the only thing that could have hit it at that range. The party monk closed a phenomenal 200' in a single round... and the storm giant just dispersed and teleported another 200' away. Still at 600'; still taking 38 damage a round. The party fled.
Legendary action teleports are brutal. Really hoping some of the other Mordenkainen's creatures have similar abilities that change the nature of the battle, rather than being big ol' bags of AC and HP and DPS.