No, they are not. Not every country requires KYC to get a SIM card. They can be purchased in many places with cash, sometimes even with default generic credentials.
Also if you add the space of emails to the space of phone numbers, it increases the space of possible sources for account creation, which logically dilutes the space.
And just to remind you, the article is about creating accounts required to use mobile phones with Google accounts. Making it possible to use your Google phone with just your phone number instead of requiring an additional email makes it specifically less identifiable.