This is a female silver-beaked tanager, R. carbo. Females/young of the closely related and similar Brazilian never has a reddish tone to the throat, and young males moulting into adult male plumage (with its bright red throat) get mottled bright red patches on the belly before they get any to the throat (like this). The female silver-beaked tanager can be somewhat confusing since it is quite variable, with some subspecies appearing almost black and others quite vine-red.
Just because someone posted it on BirdForum doesn't mean it's proven. They have their share of mistakes too (just to mention one example, check their "Slate-colored seedeater", which actually is a gray seedeater). The photo you linked is a juvenile Brazilian tanager at an early age where the sexes are identical; note its pale beak and yellowish gape. Regardless, despite its tiny size, the photo also shows exactly what I mentioned earlier: No clear red tones to throat. For a large number of females/young, see this page (search Brazilian tanager; choose "Undefined" and "female" in Gender row, and all 4 categories in Age row).
Except for the completely different adult males, there are none with a red throat similar to the silver-beaked tanager on Jackwow's photo. The distribution of the two species overlap quite extensively in Sao Paulo state, and the throat color (brown-gray, sometimes with buff tinge vs. darker and browner, sometimes with red tinge) is the feature we usually use to separate them there when no adult males are present.
