He explained that the well-known "Tree of Life" stone (Stela 5 from Iztapa) cannot be related to the Book of Mormon for reasons he explained in some detail.
Replica of Mormon Tree of Life stone
(There is a replica of Stela 5 on display in the Salt Lake Valley, described here. There was an article in the Ensign that included this statement: "If this is true—and, again, we must remain cautious and tentative until all the evidence is in—Stela 5 may prove to be the first deciphered artifact from the Nephite civilization.")
Gardner also explained that Quetzalcoatl cannot really be Christian because everything about that mythical figure that looks Christian was selected by Spanish priests who tweaked it to be a kind of St. Thomas. From that point, other writers, including LDS, tweaked Quetzalcoatl to be like Christ. However, the native Mesoamericans thought much differently about Quetzalcoatl. The legend doesn't fit.
Gardner's case was persuasive, not only to me but (I think) to most of the crowd, based on their reaction and comments I heard. In the Q&A period, some people expressed disappointment at these two corrections Gardner made. In response, he said, "Please don't be sad about good scholarship."
That is a very important concept to remember--especially for Mesoamericanists who come to realize that not only are these two long-held beliefs invalid, but so are all the other connections between the Book of Mormon and Mesoamerica (except as hinterlands). The North American model fits much better, in terms of geography, geology, anthropology, and archaeology--not to mention it fits everything Joseph Smith and Oliver Cowdery said.
True, it's kind of fun to imagine Book of Mormon events in exotic locations in Central America. I've enjoyed visiting sites down there as much as everybody else. Anyone interested in Mesoamerica ought to go visit. There are wonderful, faithful LDS living in that area that deserve our support whenever and however possible.
Just don't fall for those who make connections between Mesoamerican geography (especially specific sites) and the Book of Mormon, whether they are local guides who cater to Mormons or LDS authors who keep insisting the Book of Mormon had to take place somewhere in Mesoamerica because of the anonymous articles in the 1842 Times and Seasons.
Please don't be sad about good scholarship.