I do not think that is the strongest argument.

The strongest argument comes from Viacom v Youtube. If Viacom itself is unable to identify which videos are infringement or not how is a Youtube supposed to be able to?

Or to put into different terms. If a copyright holder historically has asked an ISP (Google eventually become one) to un-takedown content as it wasn't actually infringing; why should the ISP be liable for not-proven-in-court activity by customers?