Segregated witness soft fork: how is the funding Tx made "ANYONECANSPEND" Reading the BIP:

https://github.com/bitcoin/bips/blob/master/bip-0142.mediawiki

I see that the scriptPubKey in a transaction that funds a segwit redemption is the same as a normal P2PKH but merely prepended with OP_0. 

Is this opcode what tells older clients that the Tx is "anyone can spend" meaning that there will be no signature data in the redemption Tx?

Of course upgraded nodes will know to look for the actual scriptSig in the witness data to verify the Tx. But how does this OP_0 fool old nodes into ignoring the scriptSig on the redemption Tx?
It's not prepending a script with OP_0. It's a data push that contains the witness program hash, prepended by OP_0.

Old nodes will evaluate this as a script that just pushes two data items onto the stack (a 0 and a hash). That's obviously spendable by all, as the requirement is having a non-zero item as last element on the stack.