While my background is in engineering, I am not a metallurgist either. But I do know that 316L is considered an non-magnetic material (see https://greenwoodmagnetics.com/resource/what-is-the-difference-between-304-and-316-stainless-steel/). But even these non-magnetic grades can be made very slightly magnetic by cold working: rolling, peening, and even machining. You could anneal the work after machining, but then you'd get surface discoloration, and possibly warping as stresses are relieved. You could anneal the blank material first, and then you'd only have small work hardening from the surface machining. There is no telling what working processes your samples in the warehouse were exposed to.
TechDas, Micro Seiki's successor, uses 316L in the platters of its mighty Air Force tables. I am almost certain that 316L was used in Micro Seiki's flagship, the mighty SX-8000 II (designed by the same guy).
Another advantage of 316L is extreme resistance to corrosion. My platter looks as nice as the day Mirko machined it for me, and my heirs will inherit a shiny platter years from now! The discoloration of the original bronze platter was annoying, requiring regular maintenance, especially here in South Florida (even in constant A/C).
Let me reiterate that my magnetic attraction test with a small neodymium magnet, while not scientific, reveals only a very tiny attraction to the platter, maybe half of what even the black granite base pulls (!), and that is very weak too. That same magnet snaps with great fury onto regular steel, with a force that is probably many hundred times more. In fact it is hard to remove it once stuck to regular steel. I will attach a table of the magnetic susceptibilities of materials. Compare austenitic stainless with regular iron. Thousands of times less magnetic, even after hard working.
Bottom line, I would not worry about it. With the CU-180 mat in place, it's completely a non-issue.
As to the bearing's ability to support the additional weight, it was a concern of mine too. Mirko, who knows these tables inside and out, and who has built clones, reassured me that the bearing was so overbuilt that it would not be a problem. I certainly have experienced no problems. Remember that Micro Seiki sold several heavy accessories intended to be use on their platters, such as the ST-20 stabilizer weight, the CU-500 heavy copper mat and the peripheral weight ring. Those could add up the total weight.
As with the bronze platter, one must be very careful when slowly lowering the platter down onto the bearing shaft. The ceramic ball and thrust plate inside are very hard, but also brittle.