There is nothing to demonstrate. If I understand correctly, he is just saying anybody can create an altcoin in which the initial ledger of ownership is taken over from bitcoin.
Exactly
Of course, hardly anybody would see this as "being able to duplicate bitcoins".
The bitcoins are effectively duplicated because they can be manipulated independently in the two blockchains.
This is just a particular instance of the observation that, while there is a finite supply of "the" bitcoins, there is no limit for the number of cryptocoins. Random altcoins are often dismissed because they lack bitcoin's history; but a bitcoin clone created by such a hard fork would share bitcoins history up to the "schism". Which branch would be the "legitimate" bitcoin is not a technical question, just a political and marketing one.
Of course this is not a technical issue. o it sounds a bit like saying "passwords you can use on 1 website can be used on another website, therefore internet security is flawed".
The possibility of a hard-fork clone does not expose any security risk, it is directed against the "scarcity" claim.