I completely abandoned STD this season, but can't they still slingshot around the sun? Since the USS Discovery presumably can still use its warp drive, they can travel in time with impunity.
Yes, I know that Spock's slingshot time travel wasn't invented during discovery's original time, but Burnham in Discovery can basically do anything the writers feel like, so I'm sure she could whip up a time travel formula straight out of her rectum even if it made no sense. Hell, they can even just find some record of his time travel formula somewhere.
That said, I'm morbidly curious how this show can destroy Star Trek even more. It's literally made the rest of the franchise obsolete by showing that the Federation Falls to Absolute bulshit, what more can it do? The only thing it could do is, through time travel shenanigans, eliminate the entire franchise history, but the reboot movies already did that, it would just mean Discovery is in its own universe.
But, this is Discovery. It will always find the way to be the absolute worst. That's what happens when literally everyone involved hates the franchise.