It's all about muddying the waters and creating ambiguity, thus making future flat denials that they did the thing that did easier. The same reason his testimony before the senate was so evasive and focused on semantics.
Given that we're talking about a private conversation in this instance*, he can easily fall back on 'different recollections' or whatever.
*Never-mind that Mueller's letter was crystal clear as to what his objection was: You did not release the lovingly crafted summaries we prepared for public consumption and instead released a summary letter of your own which did not accurate represent what the report actually said, Another way of creating ambiguity.