I think you're jumping the gun a bit. They're just saying that requiring a phone number added a barrier against spammers and they want to be sure this change doesn't open the flood gates.

From the article:

"We gotta figure out some way to induce a cost for signing up without a phone number”, Kret said.

It’s unclear whether this means that registration without a phone number will become a paid feature.

However, “cost” does not necessarily mean a monetary fee; it could also refer to another barrier designed to prevent abuse.