Somebody else mentioned it, but my guess is you outfitter has probably had a few clients show up a 6.5CM and explosive long range bullets, copying something they saw on YouTube. I wouldn't worry about him, myself.
As to the Interlocks, my wife and I (and daughter) have killed a pile of game with them. My wife's .270 shoots the heck out of 140gn BTSPs, never shot an elk with it, but I wouldn't have any trepidation about it. Darn good bullet. I killed one elk it an Interlock, but that was out of a 300WM. Still, did exactly what they're designed to do - penetrate and expand without flying apart.
If you're concerned about it, you might just have a conversation with the outfitter, let him know you're .270 really likes the bullet, etc. He might explain his reasoning and it might make perfect sense. I talked with an outfitter in Helena a couple weeks ago that partners with his brother; he told me it's happened more than once that they get some wealthy guy from out of state who has literally never hunted, or even shot a gun, but booked a hunt and went and bought some high-dollar setup and hadn't even zeroed it when he got there. I'm not an outfitter, never worked in that field, but I can imagine they run into all kinds of things and clients everywhere on the spectrum of experience and knowledge, or lack thereof.