SES is an email platform. So Plunk is a self-hosted email platform - which uses a cloud platform to actually send email. If it's not sending email by itself, it's not a self-hosted email platform.

Those apps using 3rd party auth providers almost universally have them as an optional plugin, allowing you to choose one of a dozen auth providers or your own self-hosted one. You're not tied to any specific 3rd party platform, and rarely tied to any 3rd party platform at all.