I agree with respect to the low likelihood of changing one’s old posts being effective in preventing their being used as training data. I’d assume, however, that those who are motivated to “vandalize” (itself a loaded term to refer to altering one’s own words) their old posts have more than one motive; in addition to inconveniencing humans, doing so devalues reddit as a place to find information and, in theory, punishes reddit for their actions, maybe even deters others from behaving similarly.

This a situation where I think that maybe a shared distaste/disdain for “slacktivism” leads to folks discouraging potentially effective collective action in one of the limited contexts where online protest has a chance of having any effect.