It’s more like Reddit is a bunch of edgy teenagers who already skew in favor of liberal ideas far beyond that of the regular voting public. It’s why atheism is wildly popular on Reddit as well as issues like abortion or trans rights, when the American public is far more evenly split.
Yes there’s some campaigning going on on Reddit as well but it’s dwarfed by the left wing base already on the site. The swell in anti-Biden posts make this split all the more obvious.
They know the media is on them and they have to behave and say “politically correct” things.
Make no mistake, the RNC and DNC are carefully managed events. RNC tries to show as racially diverse a crowd as possible for the cameras and DNC tries to be a welcoming big tent, both to encourage undecided voters to come join the bandwagon.