You are correct. Reverse DNS can easily give wrong information
(if the bad guy controls his DNS server, he also controls the
reverse table). I know about this, but I don't care too much
about it for web access statistics.
I wouldn't care either with web stats. I was just
suggesting that as an example piece of code you might
want to add a comment about that problem/feature so
that if someone adapted it for an application which did
require checking, s/he would know about it.