Incorrect. See below. Not sure where 20 degrees is. But it’s more than idle.
steady warning horn is provided to alert the Flight Crew whenever a landing is
attempted and any gear is not down and locked. The landing gear warning horn is
activated by forward thrust lever and flap position as follows
....
Flaps 15 through 25 –
• either forward thrust lever set below approximately 20 degrees or an
engine not running, and the other thrust lever less than 34 degrees; the
landing gear warning horn cannot be silenced with the landing gear
warning HORN CUTOUT switch.
.....
sorry 737 for reference.