Oh interesting, it looks like you're right! They do appear to be using a StereoPannerNode rather than the full 3D PannerNode[0], but that is more than just speaker attenuation.
That's an old commit. It's had spatialized audio for the last month, at least.
I'm actually going to put a feature back in to let you select the audio "spatialization" model, because some folks have had trouble with PannerNode not working right. I used to have a selection between volume scaling, StereoPannerNode, and PannerNode. Been a little too busy at the day job to do it lately.
[0] https://developer.mozilla.org/en-US/docs/Web/API/Web_Audio_A...