They're talking about text messages. You can send a text message from an email address. For example "8675309@txt.att.net" would send a text message to Jenny if she were an AT&T subscriber.
I went through a massive campaign of explicit/sketchy/spam email-texts... Textra(SMS app) introduced an ignore feature that captured that thankfully after a few weeks of nonstop aggravation. My carrier would not disable that feature despite multiple requests(likely wouldn't know how either).
Yeah the first time it happened to me it was surprising and I looked it up. But now it's practically weekly. Most spam texts still come from regular numbers though. "hi /cars* compro/carros"