This article (three deep, I’m afraid!) is less interesting than you’d hope for. It’s interesting that Luther pushed the meaning of “calling” in the wrong way because he didn’t want people thinking that they had to become monks to please God. And it’s interesting that translation has been a little off on this word (particularly for 1 Cor. 7) ever since. But I’d never heard of an uber-conservative Paul who wanted everyone to stay exactly where they were at. That’s a different religion entirely. People who convert, by definition, don’t stay where they’re at.
At the same time, the guy undermines any arguments he wants to make from here about slavery because he leaps off the text in favor of “of course nobody should be a slave! slavery is awful!” To which I supply, of course slavery is awful, but Paul was also clearly not preaching la revolucion! either. Anybody who unhesitatingly tells an engaged man to stay single probably wouldn’t bat an eye at telling slaves to stay slaves.
Meanwhile, the author takes Paul to task and tells us where he went wrong. Frankly, anybody who is willing to say Paul was wrong in the text of scripture probably isn’t somebody I’d trust to properly interpret the scripture.