To be fair to enum, the services they sell are around k8, an s3-equivalent, and devops. If they sold/promised self-hosting/sovereign email services, and then were "caught" using gmail, that might be a different story.
Your point stands - they're not fully completely independent. And maybe the language in the OP's article could have been different.. but the OP also specifically says "Oh no, I said the forbidden phrase: Self-hosted mail server. I was always told to never under any circumstances do that. But it's really not that deep."
They're aware of the issue, everyone is aware of the issue. It's an issue :-) But I get your point too.
I think it would be fair for them to use something like proton or enterprise msft relay service. Actually this is only for inbound mail, it can be self hosted without any issues, spf on the other hand (outbound verification) does need a relay at minimum.