Technically, neither interpretation is mentioned in the first post. The CR analysis lothos did was partially based on "scarier than you should be", but it is linked, not mentioned. It also has the strong evidence that MitD is intended to be a force to be reckoned with, from the tower and the earthquake scenes plus word of author.
Also, how would this work? So the CR is too high... i.e. the creature someone proposes is not a challenge to the group, even though its CR would suggest it is. Why would someone suggest it in the first place? Wouldn't it make more sense to be looking for creatures that do indeed fit the tower scene? In a less metatextual analysis, why would RC recruit a creature weaker than it should be? The straight reading of the scene suggests that the problem is with MitD, not his species.
In the end, it is up to the individual proposal to justify such things. If the CR is not representative (although I'd think the more likely scenario would be a CR much too low for the proposal's actual abilities), then the proposing poster can justify this: e.g. "Sure, my idea has a CR of only 8, but it can wish itself to strength 50, so even if it gives piddly XP, it can still punch a horse through a wall" (note that such example, if one existed, would fail the tower for reasons other than CR).
Grey Wolf