that's what it says
there is a detailed review of the Type 212A class in the latest (2014) edition of World Naval Review
it says
"Fabrication..... makes extensive use of non-magnetic steel. This is supplemented by the use of lightweight glass-fibre reinforced plastics for some structures including some outer hull coverings and large parts of the fin"
The British Stainless Steel Assoc state:-
Is stainless steel non-magnetic?
It is commonly stated that “stainless steel is non-magnetic”. This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or
magnetic permeability is derived from the microstructure of the steel. A totally non-magnetic material has a relative magnetic permeability of 1. Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1. In practice this is not achieved. There is always a small amount of ferrite and/or martensite in the steel and so permeability values are always above 1. Typical values for standard austenitic stainless steels can be in the order of 1.05 – 1.1. See
Composition effects on the magnetic permeability of austenitic stainless steels
It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example,
cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.
In practical terms, austenitic stainless steels are used for “non-magnetic” applications, for example magnetic resonance imaging (MRI). In these cases, it is often necessary to agree a maximum magnetic permeability between customer and supplier. It can be as low as 1.004.