IDK where you get that statement from. 304 and 316, which are the most common grades, have very, very slight paramagnetic properties. OPs 440 might be slightly magnetic if annealed, but that's not what would be typically sold. In it's hardened state, it's not magnetic.

It looks like 440 grade stainless and is non-magnetic. It tried with a very strong Neodymium magnet. Also most mash tuns i’ve seen are either made from a cooler, copper or stainless pot and stirred with a motor in the lid, if they are stirred at all, usually just the sparging arm rotates. I don't think I’ve ever seen one with a magnetic stirrer not even sure how that would work through the grains.