Why is Safari passing this Modernizr test I wrote for WebRTC?

I wanted a Modernizr test to check for setLocalDescription with description optional. Here is what I came up with:

window.Modernizr.addTest('paramlessrtclocaldescription', function () {
    try {
        return window.RTCPeerConnection.prototype.setLocalDescription.length === 0;
    } catch (err) {
        // This can only happen if the browser doesn't support RTCPeerConnection, and
        // we have a separate test for that, but we don't want this test to crash
        // the script.
        return false;
    }
});

When I run this test in Firefox 73 (on Windows, from PortableApps.com), it properly returns false.

However, when I run it in Safari 14.1, the test returns true, even though CanIUse says that Safari does not support this function.

Furthermore, when I open the JS Console and type window.RTCPeerConnection.prototype.setLocalDescription.length, it returns 3 and window.RTCPeerConnection.prototype.setLocalDescription.length === 0 returns false.

UPDATE: The above observation is incorrect because manually running the code in the console is being done after webrtc-adapter has loaded, and webrtc-adapter overrides this function.

Why is Safari passing the Modernizr test even though it is listed as unsupported?

It turns out that Safari 14.1 does support this feature, and it seems that it is CanIUse/MDN that is wrong.

It was added to Webkit in September 2020.