IMHO what else would he say? Unless he has a Ref 5SE you can compare and he gets a sale one way or the other. Doesn't make him bad just a good businessman. .
If it is untrue, it makes him a liar. Which makes me wonder what else is he lying about? I want a dealer who will give me their honest opinion. Not someone who will say anything to make a sale. I would think twice about buying from such a dealer.
I have heard this many times before. The LS27 was supposedly better than the Ref 3. The LS27 wasn't even better than the Ref 2 mk2. It is not even close.
They are both built to a price point. The Ref series is much more expensive. It uses tubes in the power supply while the LS series does not. The Ref 5se is a truly superb preamp. One of the best preamps ARC has ever made. It will be a long time before an LS preamp comes close.