What I do not understand though, in that article, the seedlings have a brown stem, but the pines length are short (unlike mine with green stem and long pines).
Plants with too little light elongate. Pines need loads of light, which is why sowing them in winter is a bad idea. In summer light is stronger.
Timing the cutting is based on the color of the stem: As the stem matures, it will change color, going from green to purple-ish to brown. When they are this red/purple is the time for the cutting, before they really turn woody/brown.
I am not sure whether it was here or in another thread: If you are this new to growing stuff I would NOT try this technique. It adds a little to the final product, but your concern should be getting this to grow out. Next year do another batch to play with more advanced techniques I would say.