Which is to say the dealer is all wet.
Maybe he knows what he's trying to say, but fortunately (or not;) )words mean things and his words are all wonky.
Don't get premix ratio and lean/rich mixed up. As dazza said, the MORE oil there is in a volume of mix, the LESS gas there is..so that will make things 'lean' from an A/F (air/fuel) point of view. THAT is what the lean/rich issue is all about.
Certainly, too (as already said) premix ratio changes are a good way to effect A/F ratios....within reason. You're not going to want to 'prepare' for a trip to the coast (sea level...sand) by mixing up 5 gallons of 200:1 with the idea of, 'I'm rich enough NOW!!'.