This is not a discussion about what's possible, it's a discussion about what's best. You can write your own opinion here, and it seems like we're in violent agreement, but that doesn't make our opinion GitHub's opinion.

That page is just one small part of a much larger reference document, and it doesn't seem opinionated at all to me. Plus there are dozens of other examples elsewhere in the same reference that are not simple invocations of one shell script and nowhere are you admonished not to do things that way.