...One side with an alternative client and the other with their sidechain.
not true.
one side is an altcoin the other is bitcoin.
period.
I think you misunderstood what he wrote. The other admins have their own sidechant, the lightning network. And that network would have an advantage when bitcoin would turn out to be expensive and slow or not unreliable. Because transactions would swith there. Which, by the way, is their argument. We need to take out transactions from bitcoin and leave bitcoin as a high value transaction system.
Surely not what most bitcoiners would want. Or the vision satoshi had with bitcoins.
So yes, both sides have their own altcoin in mind.
Meh, as long as bitcoin does not get highjacked. They can build whatever they want on top of it.
But high value transaction system? Sign me up already.