I would say that like you I make assumptions. My assumption is simple and is as follows:
People tend to assume the worst e.g. breaking a vow of chastity would be the sexual act. What I see is clarification -not so much for himself but rather for the others involved. It may be true that many, we, do not know the specific people involved; however, they know who they are. I would contend that are is a small group including those directly and indirectly involved including innocent spouses etcetera maybe even children that know the identities of these people.
I would suggest for instance as a husband knowing your wife was unfaithful to you that the degree of unfaithfulness would equate to the pain and the burden caused the relationship. Carry this thinking out to its logical conclusion...
I would say rather than let go all those effected to the mercy of the wolves that benefit from such falls as this that the Priest attempts to lessen the blow with truth.
Those are my thoughts premised upon assumption of course...
Indeed, when we compare the situation to that of a person who has committed to marriage, transgressions of chastity include a lot of involvement short of having sexual intercourse, including things that might be known only to a spouse "wishing" for a relationship, without ever being known to the object of desire.