It certainly has a large amount of social condescension in Britain -- "not in employment, education or training" was taken up with glee by the tabloids.

Agreed. But that doesn't change the origins of the word - it just highlights the cultural (mis)representation of the term.

> Also Hikikkomori means that you literally don't leave your room at all (which is not the case for a NEET). If the hermit needs something, they'll often bang the floor to summon their mother (typically for food)

To be fair, Hikikkomori and NEET aren't meant to refer to the same people anyway. It just so happens there's overlap in the context. An example in western culture could be "freeloaders" and "unemployed". The former has negative connotations (ie they live of others like leaches) where as the latter just states they don't have a job. While some who are unemployed could be classed as freeloaders, it's also fair to say that some who are unemployed are very keen to find work and support themselves. So there's overlap in the people covered by those two terms but they don't automatically refer to the same group of people.