Except for the hand-eye coordination, a golfer or baseball batter's body technique during the swing is of no use for a tennis player because their realities are much different. Body rotation for power is required when the player does not or can not move, moves very little, or the field is large. It's clearly apples and oranges when comparing tennis with golf and baseball. Steps 1 and 2 describe moving INTO the ball with 4 steps on groundstrokes. When you move INTO the ball your linear momentum is also directed INTO the ball, they go hand in hand.
Movement equals power.
Roll the mouse over 4B to see the direction of the weight shift. The back foot doesn't stay flat on the ground, it goes up on its toe when you shift into the ball. You already know how to do this. Notice how your weight goes from one foot to the next, and how each foot moves heel to toe. The back heel lifts, leaving only the toes touching the floor when shifting onto the front foot.