They have evidence. Enough evidence to know he did something wrong and violated league policy is not the same thing as enough evidence to win a court case, and it doesn't need to be. If there were enough evidence for conviction in court, then he'd be banned from the NFL for life, I would hope.
Just accept the fact that he's violated league policy and will serve a short suspension...big deal. It's easy to stand in court, and despite proof that the injuries were incurred during the week she was with him, and say "maybe she fell, maybe she got in a fight, maybe anything"....there's no video or witness...he said, she said. But it's a lot harder to stand before a panel of folks who aren't meeting the legal standards but the work place standards and offer a bunch of possible ways it may have happened that are less feasible that DV.
Many a man has grabbed a woman by the wrists or arms and restrained her, leaving bruises. Maybe that's minor; certainly used to be viewed that way, but today, that's domestic violence whether people like it or not.
People are sometimes convicted in court with circumstantial evidence. It's certainly enough for letting an employee go. I prefer to focus on the team, not Zeke's suspension. What he did may be very very minor and may not even include striking a woman, but it seems clear to the independent investigators that he did something.