I agree, I can’t see how the shed can be 18-22 inside when it’s just a few degrees outside without a source of heat. Similarly, when it’s 30 outside you will need some cooling if you still want 18-22.
On the question of insulation though, I used 3” Kingspan in the walls, covering the door, and under the floor. I used 9” Kingspan in the roof space to keep the heat of the sun from coming through. This all does a good job but I do use heating in winter and cooling in summer.
My fermentation cupboard is built inside the shed and has additional 3” walls and roof insulation. Again I have heating and cooling to maintain a set temperature but much lower power because the space is smaller.
So considering size, smaller spaces with more insulation take less energy to heat and cool.