I have never fired a .375 H&H, but many people say the .338 kicks harder. My guess is they are shooting a "sporter" weight .338 vs. a "magnum" weight .375.
I think what you say has great merit. No matter the "perceived" recoil, everything else being equal (weight of rifle, design of the stock, same recoil pad, etc.), the "actual" recoil of the .375 H&H Magnum will be greater than the .338 Magnum.