How is that related to the article aside from "both involve bash"?
It's because rm -rf "$STEAMROOT/"* will evaluate as rm -rf "/"* if $STEAMROOT for some reason is blank or unset, which is what happened here. (As someone in that discussion mentioned, a little more Bash knowledge might have suggested using rm -rf "${STEAMROOT:?}/"* instead, to force it to (at least) error if it is empty or unset.)
So literally the only thing in common with the article is that they involve bash