They are reforesting monoculture farms and cow pasture. Given the choice between a very disturbed ecosystem full of a mix of native and "invasive" non-native trees or a very disturbed ecosystem full of nothing but invasive African grass that smothers other vegetation and impedes natural regeneration of the forest, I'd choose the trees any day. Considering the severity of the environmental crisis in the Amazon especially, and considering how much of it is due to animal agriculture and cattle grazing in particular, I'd go so far as to say that ANY vegetation is better than cow pasture.

If the people reclaiming the land from animal agriculture are going to eat, then they can either a) employ agroforestry methods in order to grow food and plant trees on deforested land at the same time; or b) allow total natural regeneration of 100% of their land and buy food grown by industrial agriculture that deforested land somewhere else and quite possibly poisoned the soil and water with -icides and then burned diesel fuel in order to harvest and transport the food.

Unless and until humans cease to exist on this planet, they will have some amount of environmental impact. Criticising vegans who are doing the best that they can to live peacefully and sustainably is not the most productive use of time and energy.

(That said, I can appreciate your concern for the ecological balance of the forest. Forest ecology is highly complex, and realistically, I don't think that any human intervention can achieve the same diversity and functional benefits of a pristine old-growth forest.)