Maybe it extended it, maybe not, my understanding is it’s hard to say.

One thing for sure: slavery lived on quite a lot more than 20 years. The abolition of the Atlantic trade was later voted to be in effect on Jan 1st 1808, the very day that it was constitutionally possible to abolish it; but that didn’t free the existing slaves quite yet. 50+ years went by to attempt to resolve the issue diplomatically, which eventually failed and gave way to 4 years of Civil War. So, that’s almost 80 years total.

But on the other hand, my understanding is no one really knew clearly what the King had in mind to do about slavery, and it was not in his interest to be too clear about it and risk to alienate either side, before actually taking action. Maybe he was planning to quickly abolish slavery indeed; or maybe just to limit it, or maybe to tax it. The Southern states were very worried they he may abolish, but I’m not sure it’s well known what his actual plan was. So, maybe he would have stopped slavery earlier; or maybe he would have regulated it the way he wanted to and then let it happen, and slavery could very well still be active to this day. No idea.