Yes it is. I thought the same when I clicked "Install App", but the big button in that popup window downloaded a tarball for linux.
They definitely need to label it better then. I know the wording, 'also available for Windows and Mac' should have explained it to me, but my brain was being dumb and all I saw was Window/Mac and I just left the site.
There was a link for Linux in the Download modal dialog.
Check again. It is.
I run the Copy client apps exclusively on GNU/Linux boxes. There is even a handy CLI (command line interface) if GUI's aren't your thing.