And how does the ingredients scale? Surely cooking for 1 person vs 4 person requires different amount of ingredients.

The % in this syntax is not to indicate information about scaling, it is just a meaningless separator: "3%tsp". I'd say both "3tsp" or "3 tsp" look better.

The author didn't go with a space because[0] some things have spaces in them, which would be harder to figure out.

[0] https://news.ycombinator.com/item?id=28999634

That is highly unconvincing! Isn't the quantity a number? Have the syntax be @<ingredient>{<number> <units>}, where the units may contain spaces but not braces. You can even make the space between quantity and units optional.

Agreed, splitting on " " isn't that difficult