I think it's only fair for them to let you know who they are.
I once "recognised" a poster on here (we've never met, but I know her DH and he'd mentioned that his DW was a Mumsnetter) so gave her enough information to let her know who I was too. That way we were on an equal footing and both had the option to namechange or not as desired.
I can't see why she felt the need to (a) namechange or (b) mentin that she knew you on that thread, though. It seems very odd.