I'm curious if anyone knows if the salt in the recipe is referring to Morton or Diamond?

Based on the fact that this recipe was created and developed in New York (where Diamond Crystal Kosher salt is readily available), I would assume that Diamond kosher salt is what the recipe creator used. According to this article https://www.epicurious.com/expert-advice/prevent-measurement-mishaps-with-this-simple-salt-conversion-chart-article, a teaspoon of Diamond kosher salt weighs 3 grams. The weights of the two salts should be the same to have the same salinity. So you could weigh out 4.5 grams of your salt and add it to the dry mixture, and that would probably be the most accurate way to calculate the difference.

Looked at online versions of the recipe, and even the originating restaurant version dies not specify which brand of kosher salt to use.
In such cases, I would use Diamond (larger crystals, lighter weight by volume) so as not to oversalt the cake.
If you only have Morton's on hand, use one of the charts or articles comparing the two brands to work out an exact volume substitution for Diamond...about half what the original amount waa by volume.