More likely a hall effect sensor, which is solid state and a lot smaller. And yes, older MacBooks had something like that, as evidenced by the fact you could put them to sleep by holding a magnet in the right place (just to the left of the trackpad IIRC in the models I'm familiar with)
Nice one ! Curious since I know almost nothing about HW - do magnets screw with computer HW otherwise ? I would guess no since we don't use HDD anymore but not sure.
As far as I know, even HDDs were pretty resilient to magnets when in their enclosures. I once took a large magnet meant for holding together concrete forms, one strong enough that it stuck to a ferrous surface it could probably support my weight, and stuck it to a hard drive for a full year to see if it'd break. The drive, as well as all of the data on it, were fine.
