In this case, I'd say that the opinion comes from knowing what the male Olympic decathlon champion could have done as a female even with hormone suppressants, so that, it seems to me, is worth knowing.
Jenner's athletic career ended decades ago. so she doesn't have before and after experience regarding sports performance. That she is a transgender female that won a decathlon gold medal 45 years ago, does not make her an expert on the subject.
Renee Richards played women's tennis in her early 40s and said that had she still been in her 20s she would have dominated and I find that to be believable.
I also believe that had Jenner transitioned when she was young, she could have won the heptathlon. That brings up the question of a whether a significant advantage in one sport justifies an across the board ban in all sports including distance running, where hormone treatments have proved to be effective.