Er, you're right, that isn't correct, is it?
The value of turn:lanes* is documented as being in direction of traveller (with the :backward key suffix then related that to direction of way):
https://wiki.openstreetmap.org/wiki/Key:turn#Example_for_a_road_with_both_directions
Been like that since I last touched it in changeset 118154623 (so might be my fault). Fixed in changeset 177150468.