Then came the word of the LORD to Isaiah, saying: / “Go, and say to Hezekiah, ‘Thus saith the LORD, the God of David thy father, I have heard thy prayer, I have seen thy tears: behold, I will add unto thy days fifteen years.’” …Clearly a different prophecy from the former, which was rescinded.
You’re obfuscating the point being made, i.e. that you just can’t spout Det. 18 as some hard and fast rule, when you have no clue if God Himself set a undeclared (by the prophet) condition that would make the word not come to pass.
In Jeremiah 18, God explicitly says He may tell a prophet to declare something, and then He may change his mind on what was declared.