As a general rule, and in view of the sign, is a vehicle driving correctly in the left-hand lane if its speed is 70 km/h?

If there were any circumstance that advised driving in the left-hand lane, it could do so at 70 km/h, but as a general rule one should normally drive in the right-hand lane.
It is justified to drive in the left lane if you wish to go faster than 80 km/h, since that is the maximum speed established for the right lane.