From what I gather then the aircraft was descended to the safe level of 1000’ on top of the other aircraft. For whatever reason he bust his level.
Although west/east levels still exist, nowadays ODLs are the norm & a lot of the time don’t need to be co-ordinated with the next sector or unit. We use them as an aircraft’s requested FL might not be available so you give them the next best level whether it is correct direction level or not, and also due to the increase in traffic in complex airspace then you sometimes need to use ODLs.