Suppose you transfer $1000 to me on Monday via ACH (the old thing - pardon my American terminology). If I'm lucky, I can wire that money out of my account, or pull it out in cash, on Wednesday. Maybe Thursday. During that time it's definitely possible you can get the banks to reverse this transfer.

If payment is instant, what that has to mean is that the funds are actually made available to me right away. If you transfer $1000 to me on Monday, and 10 minutes later I've drained my account of funds and disappeared, how can that be "reversed"? Is one of our banks going to just eat it?

If payment is "instant, but the funds are held for several days and can't be spent or transferred" then the payment isn't instant in any meaningful way.