Only 1 tsp of spice? Can't be right!

Yes it does say to make approx 5 tsp of shawarma seasoning, but the instructions only say to add 1 tsp of that seasoning to the whole dish.

Both/and.
You're both right.
The dish would likely be underseasoned if the 1 tsp spice blend were only seasoning. There is also tahina sauce and chili oil garnishes. Taste and adjust seasoning after you add those...more seasoning blend, if you like.