The source is a bit vague in this regard, but I interpreted those numbers as being the proportion of users of that platform among the 159M daily users in 2021. (For reference the total population was 214M.) So it refers to active usage, but not of the whole population.
I don’t have data for USA, but based on Orkut times I predict that they’ll vary quite a bit. (Orkut was insanely popular in Brazil, but IIRC not in USA.) And overall what I see here for Brazilians is a heavier tendency to consume [audio]visual content, in detriment of text; and yet the later is where the Fediverse is its strongest.
Regarding YT and IG Fediverse alternatives, those would be Peertube (3.4%) and PixelFed (2.4%); globally they’re a bit less used than Lemmy (3.8%), but the bulk of the Fediverse is still microblogging (Mastodon at 72%, Misskey at 8%). So if my reasoning is correct those would need to grow quite a bit, before attracting Latin American users. Or the development of local alternatives that are then “plugged” into the Fediverse, but they would need to have some killer feature.