I wonder if Ulbricht gave them Individual X's identity, or they got it from BTC-e... (Or if the NSA told them who it was and they've parallel reconstructed a plausible trail of evidence?)
If you can't hold or touch it in your hot little hand, you don't really own it.
There's a reason that many assets are described as 'real'.
What data did they have that allowed them (technically, not legally) to "seize" these funds?
According to the text, the owner of the address was involved, and presumably gave them access?