>> he also claims shooting this on DX is silly for some reason..
As best I understand his logic, his reasoning is as follows:
1. A 20mm FX lens (and all other very wide FX lenses) is generally an expensive lens. Even the old Ai/Ai-S models sell for good money.
2. If you shoot FX you have to pay the freight in order to get something approaching an ultra-wide field of view, and that is what the 20mm FX lenses are all about.
3. On DX, that is only a 30mm FX equivalent FOV, which is not exactly "ultra-wide".
4. 20mm class DX lenses are cheap, relatively speaking, with lots of choices in all the 16-17-18 to something zooms.
From that perspective, it might not make sense to buy an FX 20mm lens specifically to shoot DX in order to get the resulting FOV. But I own a 20mm f/2.8 Ai and never thought it was "silly" to shoot it on DX. It takes great images. But if my objective is to get as wide as possible for a certain expenditure, it *might* not have been the best choice.
That decision depends on the feature set and what you think of the various DX lenses, each taken individually. There are no DX primes in that range, for example. And few offerings at f/2.8, and they are quite expensive, and zooms (could be good or bad depending on your thinking).
As mentioned previously you should take anything he says with a pile of salt, not just two grains .