I saw that some people earlier in the thread were unsure if PSTP was a separate interview process or not. I also applied PSTP and saw no mention of it one way or another in my II email. So I just checked online and found this:
"[Applicants that indicated PSTP] who are accepted into the MD program through the AMCAS portal are automatically accepted into the PSTP. . . Applicants who apply through the traditional MD portal (e.g., who do not select the PSTP option in the application) and who are accepted for MD admission are also eligible to apply for the PSTP after matriculation. PSTP application following matriculation is not competitive, and we strongly encourage students to participate. Stanford PSTP’s guiding philosophy is simple – matriculate to Stanford and know that once you arrive, we will help you determine which of the many paths available will allow you to best reach your full potential as a physician-scientist."
So it looks like there's no separate process/rigorous eval for PSTP, if you're in you're in!