In this scenario, NAT was required for them to merge their IPv4 networks because of the high chance (almost certainty!) of address space collision.

With IPv6, the chance of address ranges colliding are much lower because of the wider address space, and the upstream provider wasn't supposed to have been handing out duplicate blocks. IPv6 also has it's own private address prefix (ULA) - fdxx:xxxx:xxxx:.... and if two organizations were both using it for internal addresses, there's a increased non-zero chance of a collision if they merge networks.