That's not what the guy said though. He said you can't put LVP above radiant heat because it will be a vapour barrier AND severely limit heat radiation. That is categorically false, heat will radiate through a vapour barrier without issue, and that's why we need insulation.

I absolutely agree that a concrete slab which is heated with radiant, should have insulation + vapour barrier below and around it to limit heat loss in directions you don't want, but that isn't what the guy I was replying to was talking about.