Tidebringer does not stack with Quell bonus damage
All other sources of Cleave
do bestow the Quell bonus damage onto the main target and the cleave area. In the Dota 1 version Quelling Blade
was disabled on Kunkka at 24 Jun 2011
, but in Dota 2 it was enabled from the start. The Tidebringer ability notes do not mention a special interaction with Quelling Blade either.
Since the damage was changed to be flat, it does not scale with Kunkka's item progression and there's no obvious balance reasoning to keep this inconsistency.