It is wrong to think that ‘this person did something wrong last life.’ That is not what it means. This person, if he didn’t do Aveiros, is a Tzaddik no matter what his past Gilgul did. He was put here to rectify something and that’s that. It answers one dimension of seemingly undeserved suffering.
The reason you seem insincere is because you started out as asking an innocent question while following up with ammunition, making it look like you came to fight. I understand, you simply wanted to start a conversation and started it show. But that’s probably the reason you seemed insincere.
Now, it is no secret that the only source for this is in Kabbalah. Rabbeinu Saadya Gaon does not discuss Kabbalah in his Sefarim, whether he mastered it or not. Therefore, it is irrelevant in this topic. Besides, I sort of explained elsewhere that it is not really a Stira.