In case anyone cares, I watched Burnett's video on this episode, and I can safely say that his argument is "get off my lawn." He considers the use of the Guardian of Forever (and he makes only lazy efforts to connet them) a violation of the sanctity of the TOS and TAS stories, made only worse by its implementation by a series he does not consider to be Star Trek. Burnett also lies, saying that the Guardian was never used again out of respect for Ellison, when indeed, Roddenberry himself proposed a film story that used the Guardian that would have Kirk go back in time to deal with the assassination of JFK.
That is not accurate. I watched the same video. He specifically mentions that the GoF was used again in non-canon sources such as TAS' "Yesteryear" and various novels. But he says it was never used again in canon sources, ie on screen on a TV show or movie. And he is correct about that. And Roddenberry may have proposed a story idea with the GoF but that story was never put to film. Story ideas are not canon.
Also, you are leaving out part of his criticism. Yes, he makes the "sacrosanct" argument but he also makes another argument. He criticizes Discovery for going back to Harlan Ellison's rejected idea of making the GoF humanoid. He says that while he likes Ellison as a writer, he argues that Ellison's original idea of a humanoid GoF was not in keeping with "true Trek" because it was too fantasy and Trek should be science-fiction. He quotes from the TNG Bible that specifically says that Trek is science-fiction, not fantasy. So he argues that if Discovery wanted to use the GoF that they should have gone with the depiction in City of the Edge of Forever, not Ellison's original idea.
Personally, I find the argument a bit strange because Ellison was the creator of the GoF. You would think that the creator would get a say in how the concept should be done. And yes, I get that his original concept was not used in canon. But I would think that Discovery should be respected for going back to what the actual creator of the concept wanted to do.
On a side note, I also take issue with his rant about how Discovery is not true Trek because it violates the rules laid out in the Trek bibles. RMB reads several of the rules from the TOS and TNG bibles (the documents that describe what the shows are about and how the writers should write for the shows). He reads the rules that say Trek is science-fiction not fantasy. He reads the rule that say the characters should be highly competent and people who are better than we are and people we should aspire to be like. He reads the rule that says that the characters should not run off on love affairs but should put duty first. He argues Discovery violates these rules.
The fact is that even TOS and TNG probably violated their own writing bibles a couple times in order to tell a good story that they wanted to tell. The writer's bibles are usually guidelines, not hard fast commandments.
Also, the TOS and TNG bibles don't apply to Discovery because Discovery is a different show. And Discovery would have its own bible for the writers. Discovery only needs to follow its own writer's bible, not the writer's bible of past shows.