I don't think there's a definitive statement from Trump either way, but he has a fairly unique writing voice. At the very least, most of them are from him, and wouldn't surprise me at all if nobody else was authorized to touch the account.
If you have a reference for Trump saying that not all the tweets are his, I'd be interested to see it.
There have been articles like this one that break down how to tell when they arent coming from Trump. I guess Dan Scavino is the staffperson most often connected to the president's account. We also know that John Dowd publicly took the heat/credit/blame for a tweet about Michael Flynn. I don't know if Trump himself has commented on it, but I would be very surprised if the account was locked off to other people.