They are going to argue that Biden never actually signed them. That he didn't even know about them. It's unprovable of course but it makes for great conspiracy fodder. A distraction. And that's really all Trump's handlers want from this.Of course, it's better to get it in writing for just that reason. The court isn't simply going to take your word for it that you got secretly pardoned. But that's an evidentiary issue; it doesn't speak to the validity of the pardon itself.
Obviously, in this case, it's moot, since they have it in writing.