efficient ICE cars consume ~6l per passenger per 100km.

More like 6L per 100km, whatever the number of passengers, I suppose. So it's usually still less than planes.

And there are better alternatives like trains or buses, which can be actually efficient for long distance travels (high speed trains, night travel. Works well from city centre to city centre)

There is also the additional issue of contrails which are a massive factor of greenhouse effect