I think it’s more about making things easy for his employees. His comment is just recognising that they already have personal gmail accounts so he’d like to allow them to use the same client for work email. Data privacy doesn’t seem to be an issue for him.

I do the same thing for my mail - rather than juggle between accounts I can just select from a dropdown which account to send as, and I see all my mail in one inbox.

His setup is complicated because he’s doing additional processing on the incoming mail for his domain - he can’t just hand it over to gmail, he wants to relay it. And because SPF breaks mail relaying he’s been relying on a workaround - he’ll just move on to implement RFC8617 instead now (assuming that gmail supports it - it’s still listed as experimental).