The original posting talks about "signing up" in the general context of creating an account on a site.
The article, however, seems pretty clear in talking about "signing" up to receive emails. (And very clearly puts forward that "no option == spam")
Looking at the two modes of failure for a user receiving emails you can have:
- False positives: user starts receiving email, but doesn't want it
- False negatives: user doesn't get any email, but does want it
The main debate in the original article boils down to:
- Single opt-in results in fewer false negatives, but more false positives
- Double opt-in results in fewer false positives, but more false negatives
At which point the question is one of whether it's better to optimize for fewer false positives or fewer false negatives.
In the context of the original article, if someone is signing up to receive emails, both of the following situations will lead to the original user not receiving the emails that they requested:
- If they misspell their address and the email goes to someone else
- If they enter a different address purposefully and it goes to someone else
For the user signing up for messages, the opt-in message isn't something they specifically wanted -- it's a barrier that prevents them from getting what they wanted (as such, a double opt-in request could be seen as a false positive). For someone whose email was entered in a form by someone else, any message they receive may be seen as a false positive (including a double opt-in request).