Someone told you misinfo then!!
Magnecor are as most top grade leads fully suppressed but as with all leads carrying very high current if you put them near enough to signal wires, or the map sensor itself there could be a problem, mine run within about 6" of the map sensor, and NO interference at all.
I would happily use Magnecor on mine again, only sold due to wanting different colour, the leads worked perfect, you'd need to virtually wrap the lead around the map to get a issue imo.
tabetha