Not only what you say, but this extension isn’t hacking anything, it isn’t stealing anything, it is finding an open source of the same doc if one is available and providing a direct link to the freely available open source legal to download and open document.
You are correct. Many universities have open access repositories where their authors’ works are posted, plus pre-print servers, and many draft versions of published articles are available. Unpaywall searches these legitimate alternatives for the identical (or nearly identical) content. Unlike SciHub that searches unauthorized pirated content.