That's again exactly the technical detail fallacy the comment you replied to is arguing against. No, they are not making the "odd claim" you suggest. They suggest that laws make the difference when deciding about infringement. E.g. by explicitly excluding temporary copies created while watching as intended, where it doesn't matter how the OS and the browser handle the memory internally, but a thing that results in a file on disk the user keeps is clearly different. (Similarly to how software being copied into swap-backed memory while you run it is not an illegal copy, whereas copying the file elsewhere might)