Hmm, that doesn't seem right at all. I tested the XB252Q thoroughly in my
G-SYNC 101: Input Lag & Optimal Settings article, and G-SYNC worked well below 60Hz.
Perhaps they mean the lowest physical refresh rate it supports is 60Hz in the Nvidia Control panel?
I know the minimum refresh range usually starts at <36Hz on 144Hz G-SYNC displays, but I'd have to have something like an oscilloscope to test when it starts on these 240Hz model, and I don't have one handy.
I can't imagine why the minimum refresh range wouldn't start lower than 60Hz though.
EDIT: Not concrete proof, but the XB252Q user manual states 30-240Hz range on DP:

FYI, refresh rate and scanout speed/time aren't the same thing. Just because the lowest supported scanout
appears to be 60Hz on the 240Hz XB252Q, G-SYNC should be able to refresh the screen below that Hz.
With G-SYNC, refresh rate = number of refreshed cycles per second, and maximum set Hz = how fast each refresh cycle completes. Again, not sure when the minimum refresh range is triggered on these 240Hz displays though.