Mmmm that old chesnut
Many many years ago, about 25 I think, I was working as an agricultural engineer, one of the sales guys came in one with one of "these fancy magnets" as he called them, a bloke had sold him to try, it was to suppose to increase power & reduce fuel consumption on tractor diesels as well apparently.
Anyway, to cut a long story short we dyno tested this particular tractor to get an accurate reading ( electronic dyno, very accurate )
Next we fitted this magnet gizmo & retested, fully expecting wonderfull power out, well maybe a 5% increase & what do think happened ? Yup, you got it, nothing, no difference at all.
So to sum up, you are quite correct.....an utter load of tosh