I would very much like you to be correct on this. I really miss the speedtronic limiter I had on my Mercedes but there is noting in the handbook to suggest your are correct. I've set the cruise on both my 2016 and 2019 RCs and was able to accelerate past that set speed, returning to the cruise speed when lifting off.
I feel this is something Lexus should really offer, the ability to drive as normal but not go past an upper set limit. Great for driving through average speed limit areas on motorways! Also very comforting when driving through town or urban areas. Set the limit to 30 or 40 mph and just drive normally knowing one cannot go past the set limit (unless one depresses the accelerator to the floor).