I would say self-hosting is not equivalent to DIY. It's about owning the whole stack, not about bought-vs-made.

Furthermore, the thing that complicates the current discussion is with email services in particular, so much depends on reputation and avoiding getting on block-lists. There's also the header wizardry that has become a requirement for interoperating with a lot of large email servers that isn't required for using the protocol but is practically required to ensure delivery.

On the flip-side of that, there are those who want the extra amount of customization that can only truly come from owning the entire stack. Or perhaps they already have a high-reputation IP address they have been sending from for years, and a self-hosted solution is necessary to continue with that (barring some transfer of that IP address to the operation of another company). In that case, calling this self-hosted is being disingenuous.

In some ways I see the dependency on SES (or other third party provider) as being a benefit for email hosting, especially for a new venture/property. It's why there is some value in depending on other examples of managed services. But I would be clear about that and not call it self-hosted.