There might be a false sense of privacy because even though you can post without being logged into your account, your IP is still visible to SN, so in theory SN could do a simple match between IPs and see that there are other posts with a logged in account with the same IP as an anon post.
Of course you could get a different IP to post anon, but there are other things that can be looked at that can establish a trend in the long term.