For a moment I thought you meant that this is similar to say Atom: It's an application that just happens to run in a browser engine.
The link in the article points to [1], the Chrome App Store. That in turn presents me a button 'Available on Chrome' and wants me to download that dreaded thing (the browser itself, not some Signal-Using-Chrome-As-Shell bundle) [2].
I admit I don't understand what a Chrome packaged app is, but it seems to require a normal installation of Chrome, going to the 'Chrome App Store' to finally install Signal.
That's probably what the OP (and others here) try to say: This isn't a desktop app, it requires an installation of Chrome and then might act like a desktop app if you install it that way, right?
1: https://chrome.google.com/webstore/detail/signal-private-mes...