Actually I think it is trivial for introduction points to know which hidden services they are intro points for, they just need to as clients connect to the hidden service and see if they are the introductory point selected. It is the correlation that I am worried about not MITM. It makes the hidden service less safe actually, because now not only can the correlation attack happen if the attacker owns the hidden services entry guard(s), but it can also happen if it owns the hidden services introductory points. And the introductory points of popular hidden services change rapidly because they become DDOSed. So if you compare introductory points to exit nodes to clearnet sites, in this way they would be roughly equivalent in anonymity if we assume churn time is roughly the same (new introduction point selected once every ten minutes or so). But with connection to clearnet site, if a thousand users access it there are a thousand different possible exit node selections, and if one exit node selected is bad it only effects the client that selected it, most clients will not be using that exit node. But with introductory nodes there are only a handful of them that ALL clients connecting to the hidden service with in the same ten minute period have to pick from. Now if one of the clients is using a bad introductory node they are likely sharing that bad node with hundreds of other clients all trying to connect to the hidden service.