I would quibble with this, depending on how we define "seized". My understanding is that your coins cannot be transferred unless the transaction is signed by your private key. So, authorities could seize the computer that has the private key on it, or, as you say, use violence against a known individual, but they have no way in-network to take your wallet. Compare this with the central banking system where governments can and do force banks to freeze or hand over their customers' assets.