Does this reduce Alice's ability to sue Bob for copyright infringement
at some point in the future (i.e., due to either some form of estoppel
or because she's accidentally created an implicit license)? After all,
Alice has not only acknowledged that Bobland exists, but even endorsed
it!
You've pretty much answered your own question.
It might.
Ultimately, the question for the judge or jury would be whether what Alice said did or did not grant the fan fiction author permission to make a derivative work, or ratified the legitimacy of the derivative work. The correct interpretation of what Alice said would be a question of fact for the finder of fact to determine in light of the context and totality of the circumstances.
In this fact pattern, a reasonable finder of fact could interpret what Alice said either way. Language is a slippery thing and we rely on the ordinary experience of adult human beings who are part of that language community to do their best to make sense of it on a case by case basis.
Of course, all of this analysis assumes that Alice actually owns the copyright and has the authority to grant someone permission to use the copyrighted material that she authored (as the question assumes). Sometimes that's true. But, if Alice was the author of a work made for hire and the copyright was actually owned by say, a comic book studio and not by Alice herself (and often a retail consumer or fan wouldn't read the fine print of the copyright notice and realize that fact), what Alice said would be legally irrelevant in an infringement lawsuit against Bob.
