No, it's not impersonal at all when you make specific recordings for individual clients. That you don't know who they are matters little -- going with this logic we'd come to conclusion it's not cheating if you fuck someone other than your partner but wear a blindfold while doing so, so you don't know who they are. Or if you hook up with a stranger you've never met before and don't know first thing about.
Seymour buys exclusive right to Lena's modelling content, and that's it. He doesn't limit in any manner her choices of sexual partners, or even her Stalkfap business for this matter.
Except you do not go beyond what is asked of you in the vibrator scene. Lena does what she is tasked with. It's functionally no different from eagerly sticking a dildo up her ass or a vibrator in her pussy because some dude on Stalkfap is telling her to.