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.
Then I don't quite get the point of "Wallets should be wary of any-one-can-spend scripts"