Those are valid points. But you don't need to add the square footage of the Amazon, what also happens is that forests become more dense per square foot. But not only forests, grassland sequesters about 343 gigatons and that rate has been growing at half a gigaton/year as the grassland gets taller/denser due to increasing carbon in the air. Biomass in topsoil itself sequestors twice as much carbon as biomass in trees, but this covers only 3% of the earth's surface. This paper has a nice overview[1]
[1] https://innovate-eco.com/storing-carbon-in-plants-and-trees-...
Of course that doesn't mean that reforestation isn't valuable. It certainly is. But even enormous growth of forest wouldn't make a big dent in atmospheric CO2 levels. The carbon came from underground and almost all of it will have to return underground to get CO2 levels down.