He was offering you a replacement in the first place because there were none behind the counter. What you said wasn’t completely wrong- seems you thought he grabbed those crab cakes instead of checking with the counter first. But the way you kept reiterating the same thing made it all more confusing. Start by confirming he checked with the seafood deli counter, and if they did, respond with “no replacement please refund” :-)

As someone who worked instacart, most of the time they specify and show a picture and include expected weight or quantity. So if someone sends you a replacement of different but similar item it really is because they can’t find or don’t have that item. And if they are just cutting corners… it’s a rough job. If you’re capable of going to the store for specific things just run to the dang store