@mamba brushed up against something that I was thinking about while my dogs were getting their nails clipped. There may have been some that were too weak for their CR like he suggested, but I also remember Crawford saying that the old creature format had a lot of DMs failing to see the optimal fighting ability for monsters, so the DMs were causing them to swing under their CR. I think a lot of the Monsters of the Multiverse changes were to help DMs choose the optimal fighting abilities so that the monsters swing at the appropriate difficulty. That would allow for the new monsters to be "deadlier" as you stated, while still allowing them to remain the same CR.
Otherwise those are mutually exclusive positions. I can't build an 11 CR encounter under the 2014 rules and then use the exact same monsters from the 2024 MM and have the latter be deadlier. Either the latter is deadlier and the CR is higher, or it's not deadlier.