Only a person who has Covid can infect another person. That is indisputable, unless this virus is being transmitted in some fashion they're not sharing with us.
And over 97% of the people who have Covid in the U.S. are unvaccinated. Run the math on who is statistically infecting Mexico when they go there as tourists.
I will also note that basically 100% of the tourists who tested positive for Covid in Mexico prior to returning to the U.S. (thus extending their stay in a quarantine hotel) are unvaccinated.