It doesn't mean that it will be used that way, but rather really to make it clear that Gary can pull literally anything out of his hat that the Enterprise can do, time travel included. Otherwise, why even feed us the "theory" in the first place.
To add mystery and tension to the character early in the episode. If Gary Seven revealed everything up front, the story loses the element of intrigue and character development. "Space Seed"
works in a similar fashion; Khan's true identity is only theory in the first act--nothing confirmed, so it adds mystery to a character who was compelling, but the full story was not revealed.
In G7's case, the early dialogue--from Spock's history notes & Scotty's theory to G7's vague replies about his mission were the mystery/threat set up for the audience. We do not really know much until he explains himself to Roberta late in the story, but at that point, it remains clear that time travel is not one of his
skills, despite his recognition of the Enterprise
being from the future. That last bit just as easily can imply G7's commanders have met other time travellers, but not necessarily have the technology to do the same.
Yea, but it didn't not suggest it. He could have been lying at the time just like Spock did from time to time. Changing the past could also have disastrous effects much worse than the benefits of the time changes even like we saw in that Voyager episode with Braxton where he and Janeway wound up on Earth in the 20th century and the galaxy was destroyed in the future because of Braxton or Janeway. That was the one with the Bill Gates type - Starling. So there is always three sides to every equation and story at least but it could still all have taken place in one single contemporary setting of the future or the past for that matter because it wasn't about time travel like 'Voyagers' was but about time travelers in the present, the future or the past depending on what they decide, to save money on cost.