I really doubt it.
Since the Lexus GX SUV is built on the 4Runner platform, and is a very different vehicle than the RX Crossovers, it is very unlikely that the RX and the GX are both built on the same platform.
The GX and the 4Runner both have permanent 4WD, and are excellent off-road vehicles.
The RX has always been a luxury version of the Toyota Highlander, albeit with a very different body style.
The RX and the Highlander are both available with either FWD or AWD, and are not really intended for off-road use, and as a result, I believe that the RX models are still built on the Highlander platform.
That “someone” is confused, I believe.