Alas, it's actually using the U1 ultra-wideband chip for position finding. That means this only works on the iPhone 11 and above. My guess is that bluetooth signal strength just isn't accurate enough.

You can see that this documented [1] in a footnote [2].

[1] https://www.apple.com/homepod-mini/#overview-sound-handoff-1...

[2] https://www.apple.com/homepod-mini/#footnote-2:~:text=speake...

(text fragment highlighting links only work in Chrome)