So yes, altering things - other than removal of content extraneous to the discussion - from quote boxes is against the rules. But so is falsely attributing a quote to another user. YogSosoth did so in his post. By creating a quote - inside quote marks - for Terr he behaved as if Terr had actually written those words. Terr did not.
In short, this shouldn’t be difficult. Don’t put anything inside quotation marks - or quote boxes - that you wouldn’t be able to say that person actually said. Such behavior would get someone fired at any journalism position in the country (or should) because doing so is potentially damaging to the person being falsely quoted.