Well I presume Evan's claim is that honest PoW miners will respect the propagated quorum announcements.
The whole point of instant X is that it lets you accept 0 confirmation transactions, so by the time a block has been generated by a miner, this attack has been pulled off already. If you need to wait for a block, you might as well scrap it and just use plain POW?
Afaik InstantX and Evolution are different designs. InstantX was where you presigned (on the block chain) your UTXO to a masternode so you can spend it instantly in the future with that masternode as the designated confirmer.
Evolution is where all UTXO are eligible to be instant signed by the quorum that applies to your UTXO. The quorum changes periodically (every N blocks) based on hashes from ancient history of the block chain.
My understanding is these instant confirmations are still recorded in the block chain and if they are a double-spends, they are not recorded in the block chain. That is why the block chain size scaling issue (that is causing scaling problems already for Bitcoin) is not addressed by Evolution. Evolution is not a high volume microtransaction platform even if it didn't have the other flaws I enumerated.
Since you can only spend on one quorum (or for instant x, then one designed masternode), then it is normally impossible to double-spend.
The double-spend risks comes from the holes in their design that I enumerated in my prior post(s).