It seems like this method requires a known phone number. And can track people based on knowing the phone number in advanced. That is quite a high bar, and very different from the standard stingray attack.

That is, older attacks allow you to collect all IMSI's in the area. Instead, this attack allows you to track a given phone-number, and retrieve the IMSI that belongs to a given phone number.

Edit: it seems like an Email address or twitter handle also works. What is needed is some way to trigger a message on the phone. That still requires knowing some identity up-front though.