Yep - just did a walk around with a magnet on my 2012 and concur. Just to add roof and gutter are Al..
I was surprised that the bracket that sticks out for the rear number plate and the plate next to it - both which a riveted on - were steel
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I had a 2006 MY Td5 110, Pommie built and all of the panels including all 5 doors were aluminium.
On more recent Defenders the gutter is plain steel - you see them rusting where people have had racks then removed them. I'd say the changeover was somewhere around the start of the Puma era. Before that they were always galvanised (bare during the 110 era, painted over during the Defender era).
They are coated with a very thick layer of hard-drying sealant to stop the roof leaking at the join, that is perhaps what is stopping your magnets from working.