Second, while some questions kinda sorta make sense, others are rather useless, redundant or impossible to address accurately. And some of their requests are downright audacious, not to mention grossly lacking respect to personal security. If I mine it's none of their goddamn business what the hell I mine with, what are the hardware specs and sure as hell they don't need to get invoices to every piece of hardware I mine with.
While I appreciate the idea behind this statement, they do need to cover their own asses just as you do. You can't simply say "I mined them." without being able to prove it, otherwise it invalidates the entire purpose of the survey. To me it looks as if they are trying to gather as much info as possible to soothe laundering concerns, yet still asking little enough to somewhat defend personal security.
If you really did mine the coins you should be able to show that they are relatively fresh from a coinbase tx. Most my coins only have one transaction, and that's from the well known pool to me. I'd argue that point if it were me.
If you don't like that, I believe coinbase has every right to tell you to take your business elsewhere.