Having one client, not dispositive of anything. If that client is wealthy, with numerous business interests worldwide, an attorney could not perform to the standard of “zealous advocacy” if he had other clients. Or worse, was mandated to bring them in himself.
Not so unusual.
1. He had a lot of business dealings outside his work for Trump (which is confirmed by the tax evasion charges in his plea deal).
2. It means nothing if he is disbarred, since he wasn't going to be doing any real legal work for anyone else once he burned his bridges with Trump.