I tried this, and it worked for me too.
It is not hard to understand what the problem is. By the description, the idea is to slightly push the antenna from the inside of the case towards the exterior of the case. You probably won't see the antenna inside the battery slot (as the instructions suggest) but you cannot miss it if you look outside the case. It is the small 4 cm long plastic strip that is near you on the left side when you are looking at your keyboard. This piece of plastic gets easily pushed inside the case when you carry the PowerBook, and that probably explains why it gets less efficient over time and why pushing it back in place increases the efficiency of the signal reception. There is another antenna, symmetrical, but that one cannot be reached so easily.
Portable metal cases are acting as Faraday cages, effectively shielding electro-magnetic waves, hence the necessity of designing holes for the antennas. I discovered Faraday cages when I was a child at the Palais de la découverte in Paris. I did something very close to this, and it was really impressive (OK, I admit it, scary is closer) to stand inside that cage, hands on the metal while the presenter was generating huge electrical sparks outside the cage where I had my hands.