Upvote:3
The airlines as per Timatic, which is used by airlines, states that the passport will need to be valid at the time of arrival.
Similar statement is on the page of the consulate of Mexico in Seattle:
If you are a Citizen of the United States or Canada, you donβt need a visa. You only need a valid passport.
The maximum length of stay allowed under the tourist visa is 180 days, which is likely where the 6 months validity requirement on the State Department's site comes from. It's better to a valid passport at the time of return.