The problem before was that a dash grab only worked from a very specific distance, so anytime Wario did either option from that distance, the opponent was better off moving backward a little and watching to see what would happen.
Now, if you do the shortest Dash Grab, it's as fast as the average Dash Grab in start-up frames, and the quick transition from the flashing/GO sound effect will force them to make a spontaneous decision before they can adequately process the mix-up going or perhaps even react accordingly.
Your sentence about then expecting certain things applies to doing it from a longer distance, yeah, but the main problem was that Wario basically had no mix-up until a very late frame, so the opponent had no reason to do anything but just shield/move away/whatever until Frame 24 or whatever it used to be. If it was Forward-B, they'd shield it and you'd get punished. If it was Dash Grab, they wouldn't clank and they could use an out-of-shield option on reaction. And the Forward-B slide cancel didn't do anything useful in 2.1, and that's a decently safe mix-up option to do from time to time, as well, as it can work well to avoid the shield-grab.