Definition:
Field of play: The area between the goal lines and the touch lines. Those lines are not part of the field of play.
Law 21 In Goal:
DEFENDING PLAYER IN IN-GOAL
13. If any part of a defending player is in in-goal, that player is considered to be in in-goal, provided they are not also in touch or on or over the dead-ball line.
14. If a player, who is in in-goal, catches or picks up a ball that is still in the field of play, that player has taken the ball into in-goal.
15. If a player who is on or beyond the dead-ball line or who is in touch-in-goal, picks up a moving ball within in-goal, that player is not deemed to have made the ball dead.
Law 18 Touch
TOUCH OR TOUCH-IN-GOAL
1. The ball is in touch or touch-in-goal when:
a. The ball or ball-carrier touches the touchline, touch-in-goal line or anything beyond.
b. A player, who is already touching the touchline, touch-in-goal line or anything beyond, catches or holds the ball.
i. If the ball has reached the plane of touch when it is caught, the catcher is not deemed to have taken the ball into touch.
ii. If the ball has not reached the plane of touch when it is caught or picked up, the catcher is deemed to have taken the ball into touch, regardless of whether the ball was in motion or stationary.
Therefore, as:
Law 21. 14 states If a player, who is in in-goal, catches or picks up a ball that is still in the field of play, that player has taken the ball into in-goal.
The field of play is defined as the area between the goal lines and the touch lines. Those lines are not part of the field of play.
Law 18 refers to the ball REACHING the plane of touch not crossing it.
I deduce that if a player who is standing in goal picks up a ball that has reached the plane of the goal line and then touches it down in goal then that player has picked up a ball that was not in the field of play and therefore the restart should be 22 drop out.