I doubt accepting unconfirmed Bitcoin transactions as payment would cost a machine operator much in double spends. It's too much work to attempt a double spend for a bar of chocolate.
double spends are actually easier then you think.
small items are treated as dust. so send a payment with zero fee to the vending machine. and also on a phone send the same inputs to yourself with a fee.. 98% of the time the tx with fee going back to you gets accepted. and the zero fee drops off the mempool unconfirmed.
all it takes is 2 transactions.
to make it easier. if you know the PUSHTX addresses of pools, to API call your tx (with fee to yourself) to the pool direct, and know the ip addresses of the vending machines block explorer to send the zero fee tx. you can time it right to send the zero fee tx to the vending machine. and get ur food and send the other tx (to yourself) to the mining pool to get the better chance of both "businesses"/nodes not seeing the other tx first
it doesnt require any expensive equipment. just good timing
I think that what he's trying to say is that no-one will be bothered to. But in actuality, you could even make a script for double-spending.