I’m failing to see the theological error here. The author didn’t say homosexual people are possessed by the devil or failing to use free will to some extent - just that all sin or temptation to sin or causes of sin are ultimately traceable to the evil one. Which is, like, true.
The theological error here, to the modernist mind, is that devil cannot be a factor because he no longer exists. Or if he does, he is only promoting things like intolerance and narrow mindedness.
I believe the theological error is in the bit about “natural causes disturb otherwise typical biological development”. Properly speaking those are ontological evils. A natural cause can disturb otherwise typical biological development in a number of ways - some leading to severe birth defects, some to mood disorders, some benign, some never noticed, etc.
These are a result of imperfection in the world and not diabolical intervention.
If he’s alleging the same thing for homosexuality then it would be an ontological evil. It’s unfortunate that a child is born with severe mood disorders, or no hands, or homosexual, etc. but it isn’t diabolical. These various abnormalities may or may not make living the good life harder but they are neither supernatural punishment or torment.
Saint Augustine and Saint Thomas Aquinas both comment on this in depth.