I would agree, if the app didn't say "tip for delivery person" instead of "tip for instacart" or something maybe more ambiguous like "tip." Take a look at the screenshots of their app being discussed here: https://www.forbes.com/sites/bizcarson/2018/04/24/instacart-...
Insidious, right? 100% of the tip technically does go to your shopper.
Welcome to the world of 'tipped wages'. In my state, every paycheck, the first $520 worth of tips effectively goes straight to your employer for the purposes of paying your minimum wage and then you can have whatever is left over.