Why does the Leidenfrost effect (seemingly) not apply to the case of molten NaCl when it is poured into water?
In the video, TheBackyardScientist offered an explanation for the huge explosive effect when 50 grams of molten NaCl is poured into water. He said that some water penetrated the blob of molten NaCl and blew it apart from the inside. A Leidenfrost effect on the outside of the blob is visible, but not the major effect.

The borax blob did not explode, so it must not have been penetrated by a significant amount of water. Borax is a tetrahydrate; when it melts and then solidifies, it goes thru a glassy stage which might be expected to be somewhat cohesive, perhaps resembling Silly Putty. In addition, it loses a lot of water on melting: an initial amount of 50 grams melts to only 26 grams. (Boric acid solidifies similarly.) 

NaCl does not exhibit a glassy stage on cooling (note the crystalline surface on the patty from solidified NaCl). Perhaps 26 grams of NaCl would also not explode but merely show the expected Leidenfrost effect. As a 50-gram blob of NaCl cools, solid (strong) regions form over weak molten interior; the weak part of the blob splits open, allowing water into an interior hot enough to boil the water quickly and blast the blob skins apart. The hardened skins of the blob serve as a partial container - like a pipe bomb. 

50 grams of washing soda (sodium carbonate decahydrate) will melt down to 18 grams; again, perhaps this is too little to give an explosion.

The Leidenfrost effect should be expected for small blobs of anything. The explosive effect might be expected only for larger quantities, but including even materials cooling thru a glassy stage, if the amount were large enough.