“They said Russia’s intervention and the latest advances had emboldened the regime to resist any concessions, and made it impossible for the opposition to negotiate.”

Not impossible to negotiate, it just makes the negotiating position of the rebels weaker. When you are losing this badly, negotiations pretty much consist of surrender terms. Of course the rebels are not happy with this. They would prefer a cease fire to rebuild, restock weapons from KSA/US and Turkey and improve their footing. Instead they are getting hammered and are about to lose all of Aleppo.