new here! WE recently (OMG, 6 years ago.. not that recent I suppose) renovated - and our prize product hasn't turned out as we were hoping. Got lots of questions but I'll break them all down over time

I was just wondering - how important is it to waterproof the concrete slab if laying 38mm batons for hardwood (19mm) floorboards?
The carpenter that laid our boards didn't water proof the slab, he basically fastened the buttons to the slab - as you would do with the wall frames. He said it wasn't necessary.
In discussion about some other issues, I'm now being told what he did is wrong and it should have all been waterproofed first before putting the buttons down. The batons went down about 1 year after the slab had been poured and the boards had been in our house for 2 months before being laid.
We have been here 5 years, and the boards seem ok (maybe they are drying more as I'm seeing some of the tongue and groove joins widen by a couple of mm).
Anyhow it got me thinking - when the frames are put down onto a slab, there is no water proofing, and there is no issue. Why should the slab be waterproofed for batons?
Just asking as I'm curious now.