Eh... maybe? I mean, you could achieve the same end result with a proxy/VPN and your browser's reader mode - to do it that way wouldn't be copyright infringement, would it? I feel like, in principle, Outline should be treated the same.

But on the other hand, the fact that they're transforming the page server-side, and caching it there, may push it over the line in the eyes of the law. IANAL, I don't really know.

To be honest, copyright law is such a tangled mess, I don't really give it much thought. Outline does the job, and if anyone's going to get into trouble it'll be them not me, so I'm happy enough.