The same technique will work, just don't try to push the retainer plate all the way in. As you point out, it will only go in a short way.
Using the punch, push in far enough to disengage the nub of the retaining plate from the floorplate.
Forget about trying to squeeze the magazine sides, just use the punch to pull the floorplate off the magazine forward. It will work, but if the mags have never been apart, you'll have to put some effort into it.
One additional tip you can use if you're still having trouble is to put one back corner of the floorplate against a hard surface and push the corner of the floorplate hard against the surface while you use the punch to pull forward on the floorplate.