They would probably thinks the same about the Alliance.
In fact, them thinking such was why many went along with the War of Thorns.
Still, if you want to argue how either side is to make amendments, or prove their commitment, that would include anyone suspect of willing to turn down and violate a truce.
And as far as the games history goes, the Alliance did so with the last one.
The thing is, that I do not see why would the Horde even wish to prove anything else.
They already sabotaged their own war effort in deference towards their enemies.
They ousted, killed and imprisoned those that wanted to continue the war.
They outright denounced or rebuked those that had valid reasons to want to pursue said conflict any further. To the point some were killed for it.
And they collaborated alongside the Alliance, to bring down any third party that threatened the world.
In all, the Horde has already built and cemented much more in terms of peacekeeping and compromise, than the Alliance, who still has to explain how is it that Genn went unpunished after stirring trouble during a ceasefire amidst a demonic invasion, or Jaina killing and invading the city of a third party that had yet to be involved in the faction conflict.
What compromise in terms of peacekeeping, has the Alliance done, that wasn’t mimicked in some way by the Horde?
All this talk about “the Horde must prove itself” comes of rather ridiculous and hypocritical if we are to ignore that this truce thing, is a two sides sort of deal.
To put it blunt, the Horde may start feeling willing to amend for the actions in this war, when the ones that killed Rastakhan answer for it.
Are they willing to? No? Then neither is the Horde to make any more concessions. Specially after shooting down any chance they had in this war, and spending half of it prosecuting fellow faction members simply because of how they treated the enemy.