ok, I missed that. But still, it could have been made clearer.
Why? Some of the clues were there - the lack of salt moving, and the fact the Luke avoids Kylo, but does not actually interact with him in the fight. I actually thought they were showing how light and elegant Luke fought compared to how heavy and clumsy Kylo fought, but once I saw Luke was projecting I saw that the clues were there.
The movie also showed that projections can be interacted with - the dice were held by others if I recall.
Also, Snoke can Force Choke from across the Galaxy - why not Luke?