Keep in mind that they are probably used to inexperienced people who are unfamiliar with their gun/optic setup going after something capable of ripping your guts out with a haphazard blow. They may be basing recommendations on trying to get a humane kill with a very inexperienced (with firearms) clientele. Just imagine being a guide for some businessman whose going out into the woods to take a natural born killer on a whim. He may just be looking for a mount for the "man wing" of his estate. He'll show up to go hunting wearing a track suit, loafers, and a Rolex worth more than most cars on the road with a check for $20k in hand. If said businessman becomes a mauled carcass, you don't get paid.
Look at what you have and go from there. If something you have can work, use it. That said, always check with the guide. They know the game they'll take you to. No use taking something that won't make a clean kill.
"The best caliber and round for self defense are the ones in the gun closest to you" - My Grandfather