SecureGSM was quoting a statement Roger Cicala made in response to someone who left a comment under the tear down article. If you read through the comments you'll be able to find it.How do you know? Did you do a tear-down too? Work for Canon? The rest of us are speculating.
Rodger did a fantastic tear-down. I think we can move on from what the shell is made of at this point, unless someone has definite Insider knowledge, or has cut a cross section of the shell for us.