This description of the Guardian is extremely misleading. Namely, this is only the information that can be gleaned from City on the Edge of Forever, which is not the only canon work in which the guardian appeared. The TAS episode Yesteryear is considered canon and from it we get a lot more details about the guardian. It does not merely cycle through a selection of dates, it can be told specific times and locations. Also we learn that the Federation didn't just "forget" about it, but had an active program of using it for research. Also in this episode, Kirk mentions that he went back "to the dawn of Orion civilization". Human civilization goes back at least to 10,000 BC. If the Orions are anything like humans (which a lot of Trek races are) then that would give the Guardian of Forever an upper limit of 12,200 years. And sure, the Guardian isn't big enough for a starship, but it is the perfect size for a Photon Torpedo!Guardian of Forever: Seen in "City on the Edge of Forever". Not large enough for a starship, with a lower range limit of at least a few millenia. It cycles through a list of "permissible" destinations, generated through some unknown algorithm. Doctor McCoy inadvertently used it to go back to the 20th century.
We know that the Federation is fine with using time travel when it will save the Federation, like in ST4.
Next up: the whole "many worlds" interpretation of the ST universe is a case of reaching in the extreme. It's going too far to say that the existence of multiple universes in Star Trek means that time travel is somehow governed by the many worlds theory. Time travel is well understood by the Federation in Star Trek. Officers both know how to and can do it (ST4), so it's likely that the nature of time travel would be known. If it was a many worlds situation, why do none of the characters ever act like it is? Why is there no solemn and dramatic "I'm not really back home" moment for characters when they arrive back to their starting universe? In a show as melodramatic as Star Trek something like that would happen. Would Kira want to go back in time to find out the truth about her mother if she knew she was just going back in time to meet someone who was a very good approximation of her mother? No. Would there have been so much hand wringing in Children of Time if the crew thought they couldn't go back home anyway? No. The characters don't act as if they have the knowledge they surely must have.
The final nail in the many-worlds coffin: You might say something like "The Federation may have some knowledge of how time travel works, but they don't really know how it works" at which point I say "Mwahaha you've activated my trap card!". The actions of the crew of the USS Relativity do not jive with the many-worlds theory and you can't say they don't understand time travel. Strange that Mr. Wong left out the episode Relativity from his time travel discussion. Could it be because it shits on his theories? Probably.
At this point the only recourse for a star wars fan is to say that "time travel is a last best hope" or "it's an admission of failure" blah blah. Nope. Time travel is as legitimate a weapon for the Federation as the Star Destroyer is for the Empire. There's a Q quote about dealing with more than just the mundane and learning the real secrets of the universe. I can't be arsed to look it up, but it really applies here. For all the industry and raw power of the Empire, they don't have even the slightest understanding of one of the most fundamental aspects of the universe.
Federation: Nailed it.