It is not possible for networks to "diverge to the point of no return" because they can always drop their divergent chain for the longest one.
Sure, but the issue is that the chains might have diverged so much that to abandon the chain with less proof of work would mean reversing weeks or months or payments, wiping thousands of bitcoin completely out of existence due to lost block rewards, and completely invalidating every transaction which has one of these lost block rewards anywhere in its history. The combined chaos of all that happening might mean that the people using the minority chain deliberately make changes to their protocol to prevent their nodes from abandoning their chain in the event of communications being reestablished, leaving us with two networks running side by side.
Based on commentary[1] i found, it looks looks like what Bitcoin Core do is bucket/split the node by /16 IP group.
It splits it by /16 for IPv4 and /32 for IPv6, and will try not to pick nodes from the same bucket. But as I said above, IP addresses aren't a super reliable indicator of geography, and even if they were, it would still be easily possible to pick 10 nodes from 10 different buckets all within the same country.