This is absolutely 100% a guess, but I’m thinking it’s designed such that devices automatically connect to the stronger frequency. 5GHz is stronger, but has a shorter range.
OTOH, that would only apply to devices that are attempting to connect on both frequencies at the same time.
From what I’m seeing online, unlike ‘normal’ routers, Google Mesh uses the same SSID for both 2.4 and 5Ghz. A device connecting to them may not know/notice the difference, so if it’s not physically able to connect to the 5G network, it’s not going to be able to connect at all until, as you said, you’re far enough away that it can only see the 2.4GHz network.
Granted, things are moving in the 5G direction, but there’s millions of ‘legacy’ devices out there that don’t have 5G capabilities. ISTM it would be easy enough to set up a handshake. The mesh and the device see each other, they attempt to connect at 5G, if it doesn’t work try 2.4G.
How many people are going to return the system because they can’t get their devices online…and we know Google would much prefer you connecting through their mesh system than a linksys or netgear system.