This is self hosted. I think we'd all accept that a "self hosted Stripe" that obviously had to offload payment processing to a third party gateway could still be self hosted because the platform is, not every dependency. Volume email is not something you should be looking to 'self host' and would leave the product dead on arrival if it did.
It feels like pedantry to critique that aspect so heavily over spirit v letter.
This is one thing to have a self hostable platform you can plug to any email infrastructure of you choice, another one to have it dependent on one specific vendor.
Agreed. I've been self-hosting my email with Gmail for 15 years and it's perfect. /s