replacing carpet with 5" x 3'4" brazilian cherry pre-finished solid wood on first floor. Lumber Liquidators sold me the QuietWalk underlayment. I'm reading it's it's used for only "floating or laminate" floors? My installer seems to think it not a good idea?

thanks

Your salesguy at LL was just doing a remarkable job of upselling to an unsuspecting retail customer.
Quietwalk is for sound attenuation under floating floors. Once you nail through it, it loses its design purpose.
You will get some reduction in reflected sound (for this purpose we use cork) but Quietwalk wouls still be alright.
Did he really say you NEED it?
Anyway, if you cant take it back, you may as well store it for future use, or flip it to a friend who is installing a laminate or other floating floor.
I dont quite agree that its a bad idea, but certainly serves no useful purpose in your application.

Tell your salesmen to put it under his hardwood , and get yourself some 15 lb felt ,or better yet some aqua bar B .

No, do not use it under solid wood. It is undercushion, for floating flooring.