The idea is that we shouldn't go vegan because land in third-world countries is used to make: avocados, mangoes, etc.
The mass deforestation in South and Central America is from animal agriculture. So, yes, avocados use some land as well, but if you stopped eating meat, the amount of land usage would, in net, shrink. This is a problem that animal agriculture has created and therefore is a circular argument.
You can't attack veganism by attacking a particular food. That doesn't suddenly make meat more sustainable. If you think that you should not eat a particular food on ethical grounds, even if it doesn't contain animal products, then great! Do that!
The forests in this part of Mexico are being cut down for a variety of reasons. One reason is that the timber industry is booming in Mexico and there is a high demand for wood. Another reason is that the Mexican government is encouraging the development of agricultural land, and many of the forests are being cleared to make way for farms and ranchlands. Finally, some of the forests are being cleared to make way for roads, homes, and other infrastructure projects.
Cattle ranching has reduced the number of high forests in Mexico by more than three-quarters since the turn of the century. The destruction of pre-Conquest cities like Palenque and Teotihuacan is linked to deforestation.