Not always; it's only true when each IP = one editor, in which case it is indeed less anonymous than pseudonyms.
Pseudonymous editing can be much less anonymous - consider if you are editing from behind AOL's dynamic IPs, or from one of the IP addresses that proxy entire countries. That is, if you're a Qatari editor, editing anonymously means your edits could belong to any subset of ~1.5 million people; if you edit pseudonymously...