In this situation, before you overtake the cyclist, are you obliged to sound the horn?

Why?

There is no general requirement to sound your horn before overtaking a cyclist, as the horn is used to warn of danger. In the video, the cyclist is riding near the right edge, and the oncoming vehicle means you must wait for safe overtaking conditions. A specific duty to give short horn signals applies to motor vehicle drivers when overtaking or passing an obstruction in fog outside a built-up area.

art. 29 ust. 1 oraz art. 30 ust. 1 pkt 1 lit. b Prawa o ruchu drogowym