Yeah - assuming this was not the rightful owner (which it might well be), my gut is that perhaps someone found an implementation flaw/quirk in some old wallet c

Yeah - assuming this was not the rightful owner (which it might well be), my gut is that perhaps someone found an implementation flaw/quirk in some old wallet code/keygen/RNG that effectively reduced the keysize down to something more manageable for brute-forcing. In ye olden days Bitcoin was still something of a curio for geeks and nerds and not the industry it is now, so it would not be unreasonable IMO for there to be some slightly-less-than-perfect implementations floating around from hobbyists or open source etc - the stakes were lower then. If there was say a vulnerability in a specific wallet version it would be quite possible to narrow down search space to only the wallets/addresses around that point in time etc as well, making it easier to target your brute-forcing efforts. It will be interesting to see if any other dormant wallets from around the same era wake up too.