So they'd seized a safe that contained a note saying where the money/Bitcoin were hidden, but they didn't have the combination to the safe, and they didn't know if anybody else had a copy of the note.
Calling those "seized bitcoins" is definitely stretching the truth in my opinion. And the Government,ent/prosecution knew this - from the article:
"As a backup, Trezor hardware wallets can generate a “seed phrase,” a combination of as many as 24 words that can re-create those private keys on another device. In essence, anyone who knows the magic words and an additional PIN can take control of the Bitcoin. Unplugging the wallet device and physically locking it away is no protection. Brown warned that Larry could remotely take Bitcoin and that the government would be powerless to stop it. “Until we can secure them and transfer them to a government wallet, those are available for him or his family members to transfer,” he said in court. US District Judge Beryl Howell granted bail anyway."
Having said that, the brother was an idiot for ever thinking he'd get away with it. Unless he was going to torch his houde and fly to Belieze before the cops caught up with him, he was bound to get caught. As the article posts out:
"An informant told them Gary had asked his advice on Bitcoin gambling services, records show. The source believed Gary wanted to use them to mix Bitcoin he took from Larry. Gary is “not the sharpest tool in the shed and did not think through the consequences for his brother” before he moved the Bitcoin, the informant said."