"The fact that the Nephites and Mulekites remained oblivious to each other for so long implies extraordinarily difficult terrain between them."
As he points out, according to his model, these people were only 320 kilometers (about 200 miles) apart. In 400 years they're never going to run into one another? When they were both part of a far larger Mayan population? Kirk ought to rethink his model; i.e., he has the two groups much too close together.
One aspect of this that undermines the whole Mesoamerican theory is that the Nephites found it remarkable to discover Zarahemla, and Zarahemla was excited to be discovered, but they were both supposedly living among long-established Mayan cultures. If any Mesoamerican proponents have explained this, I haven't seen the explanation yet.
Here is Kirk's overview of Book of Mormon lands:
If only he would apply his expertise to North America, we'd be getting somewhere.
The biggest stumbling block for Kirk is probably the River Sidon. Here is the Q&As he provides, with my comments in bold: