This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia by just maintaining a large number of network connections, which is more of a burden on the network than double spend relaying would have been.

Now you will often hear straw man
arguments like “only the blockchain can prevent double spends, that’s why we have it”. Of course, but notice that we aren’t talking about preventing double spends, only detecting them. And in the six years bitcoin has been going, this risk mitigation strategy has worked fairly well. Most bitcoin purchases are done through payment processors who accept zero confirmation payment...
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