As mentioned in the article, even after acquiring the wallet file, the FBI is still unable to perform transactions with that BTC. There's no bank they can order to transfer it into their account, no briefcase full of cash they can deposit, etc. As I understand it, the only way they're able to deprive Ulbricht of them is by not letting him keep a copy of the file. With the "brain wallet" strategy (not sure how viable that really is), they wouldn't even have been able to get this far.