It's currently also true on Android, because Gecko doesn't yet provide a WebView style embeddable component, so Firefox Focus uses the default Android WebView instead.
It is also the case on Android. It's nice, because many people will want to install it alongside a full-blown browser. And it makes browser startup quite a bit quicker, making it very much effective at this one-off search use-case.
And it happens to block the vast majority of ads, so Mozilla isn't exactly making friends with webpage owners by supplying this. Had they used Gecko for this, then the percentage of people on Gecko with ads blocked would have risen and that might have resulted in webpage owners spending less time to ensure that their webpage isn't broken on Gecko, therefore effectively dragging down the situation for actual Firefox.
There's a bug tracking work to migrate to Gecko though: https://github.com/mozilla-mobile/focus-android/issues/13 (see also the linked https://bugzilla.mozilla.org/show_bug.cgi?id=1322573)