Good question! Currently, using a Google Home Mini as a hub for matter compatible devices is not supported. We're always looking for ways to improve -- we'll take this as feedback, and we'll gladly pass it along to the team. You can also send feedback using your device by saying, "Hey Google, send feedback." Visit this link for more steps.
Here's a link that might be helpful to you: Google devices with Matter
the key part of that link is this:
however it is unclear to which degree each device supports which part of matter.
furthermore Apple's speakers ARE Homekit hubs
furthermore it also does not indicate any of google's plans
For example. I have spent a small fortune on a Google nest doorbell, 6x Google Mesh wifi points, google speakers and so on. yet it seems that my Mesh wifi (not the pro version) and doorbell (gen 2) are incompatible with matter.
am i seriously expected to replace all this? it will cost me hunreds of pounds and as an iPhone user it just tempts me to go all-Apple