Intensity minutes ver.20.01 and extrnal heart rate measurment (bike rides)

Here’s the Garmin advice - https://support.garmin.com/en-US/?faq=pNU9nnDzzGAHmEavp9rpY8

Key point is that intensity minutes are only recorded when heart rate is moderate or higher for more than 10 minutes.Check your heart rate zones are correct and ensure you are active at moderate intensity or above for more than 10 minutes. The count will stop if you reach say 9 minutes then drop out.

2houers bike ride with avatage 160 bpm heart rate... 

You must maintain 10+ minutes straight that will show above the white line you see in your example, to receive Intensity Minutes.

but that doesn't make sense because the white line is average. According to this logic, 2 hours of activity are non-intensive.

Intensity minutes are coming from your extended periods of time 'above' your average. The white line example is not a perfect example of the complex algorithm being used to determine intensity minutes, but looking at your example, you will need longer sustained periods of time to receive more intensity minutes if that's your goal.

Please refer to the article that  philipshambrook provided. It explains it best. There is no additional information outside of that Support Center article.

This approach means that interval training with a large variable heart rate will not be minutes of intense activity. likewise, if the variability of the heart rate will be at its very high heart rate, however, at its frequent variability. 

I think that with older versions of the software (ver.19.77) there was no such problem.