Is it possible to extract legacy address from segwit address?
While technically it is possible to generate one address from another, after all they are simply representations of public keys, it is strongly discouraged to do so, if you are not the owner of the address (at which point the wallet should provide you with an option to use either format). The problem is that if you send a transaction to a segwit address, that the recipient is not expecting/supporting then they will not recognize the payment.

Now for the apparent aliasing you mention above: the recipient address of the transaction you link to is a P2WSH (pay-to-witness-script-hash) address. cdbf909e935c855d3e8d1b61aeb9c5e3c03ae8021b286839b1a72f2e48fdba70 is the hash of a script that currently only the owner knows (until an output to that address has been spent). The bc1... representation is the bech32 encoded BIP142 address, which includes a checksum and a human readable currency identifier bc. It contains exactly the same information, i.e., a P2WSH script of the form `0 .