The other way to look at it is that even if the 3P SIM folks did it prior to 60 days and don't receive GCs, that just means the Google Fi viewed that their service was not "active" for 60 days, and therefore they didn't comply with promo terms, even if they continued paying the bill for 120 days (i.e. not "cancelled").
Conversely, if they DID insert 3P SIM and received, then Google Fi viewed that their service as active anyway.
This reading, which may be the better one, would not require one who hasn't yet inserted a 3P SIM to wait until 120 days to insert.