As to why Ligon was allowed to preach, why not? He wasn't applying for membership at CHBC.
Baptists are in the practice of allowing those who are not "members of the visible church" to fill their pulpits? Do they, perchance, have a higher standard of the Lord's Supper than the ministry of the preaching of the word?
It's not normative. I know the Duncan and Dever are friends. Both have appeared together at the Ligonier conference. I can't answer for Mark Dever as to why Lignon Duncan preached at CHBC.