GPS jam uses data from aircraft ads-b GPS accuracy data reported, so they should have global air traffic route coverage. Would be interesting to fuse that data with this data (sensor fusion).

I am quite skeptical at least with the low interference. My entire country, NZ, the whole of Europe, India, USA and most of inhabited Australia and Canada are reported having low Interference. Is this likely to be true?

Yes, because jamming GNSS in those countries incurs penalties from RF regulators (broadly speaking). Consider the power output required in order to impact gps accuracy at airliner cruising altitude (~9-10km above ground level). Your RF emissions will not go unnoticed by those responsible for ensuring RF hygiene within their jurisdiction.

I think I misinterpreted the map. So "low interference" on this map means "probably no interference" not "low levels of jamming"?

Because i agree with you, nobody is jamming on a large/noticeable scale in New Zealand.