It depends on how the cut was done/intended to be used. It's not unusual for a cut to be custom for a particular sight - the slide has screw holes drilled to specifically line up with the sight. In this case there's no intent to use a plate - the slide and sight match each other already.
With the Glock MOS versions, the slide screw holes match up to the plates. The plates are different to match to the different sights (most fit a couple of standards). This gives more options than the typical custom rmr cut.
So, with that said, if your Holosun matches up to the slide you can screw it directly onto the slide (be sure to use screws the correct depth). If it isn't a match to what's already been done to your slide, you could have some challenges. You would either need a plate that matches your slide and the Holoson (think converter) or you would need new screw holes put into the slide. Personally, I would seriously consider just using the RMR the slide was originally cut for. You mentioned the Holosun was the same pattern as the rmr so hopefully everything lines up and you just need the right screws and a little locktite.