If you actually read the bug, you'd see it was a request for an option that opening the keyboard tray not wake up the device, which it does currently.

If the software is changed to not wake up the device when the tray is opened, it presumably will not wake up when a parasitic magnetic field passes through it, either.

But as for parasitic vs. non-parasitic fields -- it would actually be possible to tell them apart if they were modulated by the hardware, e.g., frequency detection. Instead of a fixed magnet, an electromagnet would emit a small signal which the Hall Effect device could detect and demodulate to distinguish it from background noise.Of course emitting the modulated electromagnet would use up more power.

(OT: I once submitted a bug against a paper shredder company whose shredder thought that the bin was full whenever any light passed through the bin window, and refused to shred. I had to tape up the window to keep it from picking up parasitic light. Had they simply used a modulator-demodulator for the LED transmitter-receiver pair, they could distinguish parasitic light from their own LED's.)