If that combined with a specific name drop are "uncertain" to you, you should retake your English classes.
GitHub qua "template engine website" is "NOT permitted". That's certain.
GitHub qua code code host? Not only is that, in fact, not stated "in no uncertain terms", but insisting that it's both forbidden and that it's clear that it's forbidden comes across as trying to will your way to truth.
Read that rule again, specifically "sites /that/ generate HTML" (emphasis mine).
You are not permitted to use sites that generate HTML, period. Even if you use a "site that generates HTML" like Github only as a file repository, that is still against the rule.
Your argument would be fair if the rule read like "using sites, such as Github, to generate HTML are not permitted", but that is not what the rule says.
> The rules don't specify "github.io" or "load .md files", it broadly specifies "Github" the website "that generate[s] HTML from text" in no uncertain terms.
And that's just not true; the no-GitHub-whatsoever interpretation that you're pushing simply can't be described as "no[t] uncertain". In fact, I'd wager that a polling would reveal that most folks in a position to interpret the rules would come away quite certain that it isn't forbidden to use GitHub qua code host.