802.11n can run on the 5 GHz band, and HomePod mini does indeed connect to 5 GHz Wi-Fi networks (Like other S5 powered devices).
I know this because my 2.4 GHz and 5 GHz networks are named differently, and mini connects to the 5 GHz network. A quick web search will also support this.
OK, nothing to do with antennas then.
The change must be due to switching to S-series (ie: Apple Watch) SoCs in the HomePod mini and HomePod 2nd-gen, which have built-in 802.11n WiFi.
The original HomePod used an A8 SoC and would have had a 3rd-party WiFi module with more capabilities?