By the by, the Sermons were outdated before TES III was released and so the contradictions between the Sermons and the game are numerous. They shouldn't count as legit references.
That excerpt goes against what all he said to the Nerevarine after the MQ about things like rebuilding the Temple and helping the people. Maybe he wrote that when he was a younger more spiteful false god in order to propagandize and protray himself and his other triunes as all-powerful early in their millenia old regime. Maybe he sobered up and grew a conscience. He doesn't come off as the type of false god that would destroy his people if they stopped worshipping him. Indeed if I remember he implies that they should no longer be worshipped as gods but revered as Saints. He states that he loves his people and he designates the Nerevarine as the Protector of Morrowind. What better way to protect Morrowind then to neutralize that rock.
Even still, if the Sermons were legit and indeed Vivec was truly malicious, why wasn't the topic of landfall mentioned once in the Nerevarine's many conversations with the denizens of Vvardenfell. No one says, "Yeah if we stop worshipping Vivec, he'll annihilate us with the Ministry of Truth. Yep...svcks...gotta love the man." All in all, quite a sorry epilogue to TES III. Morrowind spared from one disaster only to get destroyed by an even greater disaster--from the frying pan into the fire...FAIL!
If Vivec suddenly became less of a dike, then why not get up and get rid of Lie Rock? Because it's being used as a prison? He's a god for crying out loud, he can build a new canton and put all the prisoners in it, then get rid of Lie Rock. Nope, I'm positive his intentions were always the same, and that is "If they stop loving me, they're going to die."
Edit: Besides, if he TRULY loved his people, he would never put them in that much danger. The fact that Lie Rock was still floating there shows that he couldn't care less.