It's worth investigating. I think the main reason he hasn't just said it is because it's a highly speculative thing with his current data set. El Mirador is where most of his work is but El Mirador is not as old as Highland sites like Kaminaljuyu.

It would be a bit frowned on for him to make that speculative a claim without more data. Issue is collecting data in this field is very time consuming and very expensive.

EDIT: He also apparently did say it and has been frowned on outside my knowledge.