Does this mean that by default anyone-can-spend scripts are disabled and cannot be used?
AFAIK it's the opposite.
Since segwit is a soft fork, what would happen if a miner treats some of the segwit outputs as anyone-can-spend scripts (as pre-seg scripts) and spend them in a new block?
Most Bitcoin full nodes (whether it's run by miner, exchange or regular user) would reject such block because there's no valid redeem script or the UTXO is invalid.