Well, Uncle Rick had said at the time, when asked why the Guardian of Forever wasn't used, he called it a "cheap plot device."
Yep, that "cheap plot device" from arguably the most popular episode of the original series, which also won a Hugo award and was penned by one of the greatest SF writers of all time.
His answer: to come up with a cheesy way to do pretty much the same thing, albeit with no "rules." Because hey, when you're killing off a legend, you may as well ignore all the good stuff that came before (that wasn't created on your watch).
I've purchased the riff, but haven't had the chance to watch it with the film yet. I can't wait.