Yes, Focus uses WebView. On newer Android versions it's actually provided directly by Chrome (Android 6+ IIRC), on older Android's (Android 5) it's a separate package that's still built from the same sources. (Android 4 is more complicated but also not supported by Focus for that reason.)
Interestingly Firefox Focus uses WebKit (/Blink?), since that's what Android's webview provides. Firefox for Android should get the improvements around version 58 or 59.
Also good to note, for people planning to stick with an older Firefox for as long as they can: Firefox 52 is an Extended Support Release, and as such will actually be actively supported until June next year. This will give WebExtensions some extra time to mature for more use cases: https://www.mozilla.org/en-US/firefox/organizations/