A lot of the times I get the impression that they're saying it bait people, or to give their posts an ironic tone of voice, (although I get this impression more when people misuse thicc). Other times it seems like the person geninuely believes that chubby (or thicc) is just a generic term for fat person, which wouldn't be surprising. In real life and communities outside of 4chan, chubby has used both as a euphemism and an outright synonym for a fat person of any size. It's a tired word that has lost a lot of meaning. There's a good chunk of people who use the word properly, but there's also a good chunk of people who don't.