Amazon is unique the way they work because they sell so much stuff and anyone can go and list something for sale.

Labeled as "FSA Eligible" is really just within the vendor's internal system. If you tried to buy that item with FSA debit card it would not go through. For reimbursement it doesn't matter

All that matters is if the item really is FSA. Your FSA is going to check if the item is eligible per the IRS rules, they are not going to check if the seller marked the item as FSA or not. In fact the seller could make an error and your claim would be denied because the item does not qualify as FSA item.

For example I bought a BMW first aid kit for my car from a car parts store and then submitted reimbursement from my FSA without any problem.