I know I benefited from "newbie gains" since I hadn't lifted seriously in ~20 years.

This is honest and important context. The lean mass gain you're seeing is partly muscle memory (satellite cell-mediated re-growth of previously trained fibers) and partly true newbie gains. A trained individual in the same situation would not see 5 lbs of lean gain during a 40-lb weight loss.

But here's the thing: most people starting GLP-1 therapy AREN'T trained. So your results ARE representative of what an untrained or detrained person can achieve if they commit to resistance training from day one. That's the message worth amplifying.