I agree with you. The reason for the defense having a higher PPG for Leach, aside from the conferences he's coached in being more offense oriented, are his QBs throwing a lot of INTs and the offense scoring a lot.
Those two combined puts the defense on the field a lot. Leach should be able to get a better quality QB at MSU than he could at WSU, so hopefully INTs are less prevalent than his past WSU QBs. Even if our QB doesn't throw INTs, the defense will get on the field a lot, so more opportunities for the other team to score and put up some garbage points especially.
PPG allowed probably isn't going to be our barometer for success. It's going to be INT thrown by our QBs.
Some WSU QB stats and team record:
2019: 6-7; 16int
2018: 11-2; 10int (Minshew had a 38-9 TD/INT ratio)
2017: 9-4; 20int
2016: 8-5; 12int
2015:9-4; 12int
2014: 3-9; 18int
2017 looks like an outlier. The starting QB (Falk) had a 30-13 TD/INT ratio, but the backup Hilinski had a 7-7 ratio. There may have been injuries or something this season for WSU, bc Hilinski played 3 meaningful games and he threw 6int in those games (4 in one alone), so I think all this data backs up what I said about INT thrown by our QBs will be the ultimate factor for how much success we have.