Answering again: The next witness in that round will immediately reject the block and make witness one miss a block. After 24 hours of missed blocks that witness is automatically disabled. And we have alerts for missing blocks and will by then already have multiple witnesses screaming that that witness is hacked.
(answering all this as a steem witness, bts is similar but can have subtle differences)
RE: Some weaknesses in either the DPOS algorithm or its explanation