The fallacy is "if it can be trained, it is therefore logically a muscle".
Ask your same dictionary to define muscle, and ask yourself if the immune system is a "band or bundle of fibrous tissue in a human or animal body that has the ability to contract".
Except my statement was "If it can be trained, it _behaves like_ a muscle", meaning that muscles and the immune system can be trained.
I don't see how it can be reasonably misinterpreted as "the immune system is literally a muscle", I'm not talking about unfeathered chickens here.