I know how to write XML by hand. It's not hard. The reason I use Jekyll instead is because updating the RSS feed is something I might forget to do at all. Before I used Jekyll, it was a custom Perl 5 script, and if GitHub ever stops supporting Jekyll, that's what I'd move back to.

A well-designed SSG, like all of the ones I've used, will interlock[1] updating the feed and the HTML. You can't easily update one without updating the other, just like you have to go out of your way to open a microwave oven without also turning it off.

[1]: https://en.wikipedia.org/wiki/Interlock_(engineering)