Ok, just read his email and I hated it. I usually like him pretty well, except for his anti-scientific beliefs around autism. However, it rubs me the wrong way when people impose the idea that forgiveness is the only way forward. Maybe I am wrong or projecting because that is a personal pet peeve, but that's how I interpreted his email. I also don't think it was a good call on his end to bring up that an anxious parent + an anxious child is a bad match at the same time that he pushed his forgiveness approach. I can totally see why you felt like he was letting your mom off the hook. It feels like he's empathizing equally with your mom as with you. This is actually an example of countertransference, I believe, and I find it disturbing. I think that he is empathizing with your mom because he also finds you difficult at times due to your anxiety - though he perhaps won't admit it even to himself. Because he is not very educated in transference/countertransference and projective/introjective identification, he is very unlikely to recognize what is going on. Here is an excerpt from this website that I think is highly relevant.

"When they become recipients of what their clients’ project – sometimes called projective fantasies – therapists identify with their clients and/or with those who have been significantly impacted by their clients. Grayer and Sax (1986) note that in any given session, the therapist usually moves back and forth: identifying first with the client, then with a person who was impacted by the client, then with the client, and so forth."