No it can’t. https://wiki.mozilla.org/Mobile/GeckoView: “GeckoView serves a similar purpose to Android's built-in WebView, but it has its own APIs and is not a drop in replacement.”
The purpose of WebView has certainly never been about using a different engine; rather, it’s to have only one copy of a browser for apps to use as a widget, rather than each app bundling its own browser which uses vast amounts of space and raises serious security concerns.
The purpose of WebView has certainly never been about using a different engine; rather, it’s to have only one copy of a browser for apps to use as a widget, rather than each app bundling its own browser which uses vast amounts of space and raises serious security concerns.