Assume their recommendation is not aimed at you, but at the 99% who like the idea of running their own mail server...but lack the skills and commitment to do that securely, over time.
DigitalOcean might not be the target of the resulting spam tsunamis, but they still bear a fair hunk of the "support & consequences" costs.
DigitalOcean, or more precisely - some of their customers are the source of quite a few spam tsunamis, at least according to spamtrap data I'm collecting on my mail server. So from that perspective, a big "thank-you" goes out to DO for firewalling off outbound port-25 traffic by default.
That said - reading "Even if SMTP were available" and "recommend using SendGrid", it sounds like there is no opt-out from this. Even for customers who would want to self-host a mail server that never sends in bulk. Now that, is reprehensible, and adds yet another brick in the already tall wall that mail duopoly+ (gmail, outlook, and handful others) enjoy.
DigitalOcean might not be the target of the resulting spam tsunamis, but they still bear a fair hunk of the "support & consequences" costs.