Apparently you missed the entire first clause of the post you are quoting, about true prophets not attempting to sell God's revelation(s). The fact that the revelation didn't come to pass isn't the point, though it is another nail in the Mormon coffin.
And besides, when Hezekiah heard those words, he wept bitterly and prayed to the Lord, telling Him that he had done all that was good in His eyes. So the Lord heard Hezekiah's prayer and extended His life. Is there really any parallel to this in the failed sale of the BOM copyright in Canada? I wouldn't think so, as it is not about JS' life or any of the travelers who attempted to do the actual sale's lives. It is about a commercial transaction, and is written in such a way as to
allow it to fail and JS and Co to still save face. Is this the case with the revelation to Hezekiah through Isaiah?
Verses 4-6 don't make it out to be that way. For one thing, God doesn't even allow enough time for anything to have already transpired before He gives a new message to Isaiah:
4 And it happened,
before Isaiah had gone out into the middle court, that the word of the Lord came to him, saying, 5 “Return and tell Hezekiah the leader of My people, ‘Thus says the Lord, the God of David your father: “I have heard your prayer, I have seen your tears; surely I will heal you. On the third day you shall go up to the house of the Lord. 6 And I will add to your days fifteen years. I will deliver you and this city from the hand of the king of Assyria; and I will defend this city for My own sake, and for the sake of My servant David.”'
So whether the original message was conditional or not doesn't really matter, because Isaiah doesn't even have a chance to leave before He's told to go back and tell the king something else. So there couldn't have been an 'after the fact' rationalization as there is in the JS' failed revelation. It was instead essentially a warning "Get yourself together, or you're on your way out", and King Hezekiah did the right thing by responding to it
immediately. It's not really comparable to the BOM in Canada situation then, is it? It's not like JS gave the revelation
to the Canadians so that they may react to it (or if he did it's not clear that he did so; I'm getting my information on this matter from
this Mormon apologetic website, which does not mention that), which is pretty weird when you consider that in the Bible, God's command is at least aimed at the people who need to hear and respond to it, and yet that does not seem to be the case here.
In fact, now that I think about it, why does God give this revelation to
JS in the first place? He's not even going on the trip (which, oddly, is one of the criticisms the page gives of the people who brought it up later to cast doubt upon JS -- "so and so wasn't even there"; neither was JS -- does that mean his own conveying of the 'revelation' is therefore not to be trusted?). The revelation as you can read it at the link mentions Oliver Cowderey, Joseph Knight, Hyram Pagee, and Josaiah Stowel. Is God unable to speak to anybody unless he clears it with/through JS first? Hmmm.