It does very much depend on the tyre and rim combo IME. I too fitted Schwalbe Ones onto my Pacenti SL23 rims without the use of levers, however try doing the same with some other tyres and you may be disappointed. I snapped a tyre lever with some IRC RBCC’s, and Hutchinson Intensives were a struggle too onto those same Pacenti rims.
There’s a video (GCN I think) of the new Mavic tubeless wheels/rims where they talk about how some tyre manufacturers err on the side of caution on the small side and some rim manufacturers go the other way. If you get the extremes of the two you may well struggle, which is maybe what I found.
YMMV may vary obviously! Nowt to do with technique either, once the IRC’s were “on” to the rim awaiting seating (that’s another story again!) you couldn’t budge them for love nor money. They are just a very tight combo, simple as that! 😉