A few comments from someone who regularly trades on PredictIt:
1) The maximum allowed bet is $850 (not $800), and this is only on a per contract basis. Therefore, if you are betting on Biden winning, you can max NO on Trump and YES on Biden for a total of $1700 in the same market.
2) PredictIt also has numerous overlapping markets which are all tied to the results of the general election. Currently all the markets below let you bet on the outcome in November, allowing your max bet in practice to substantially exceed the $850 limit.
...and this doesn't even include state specific markets or markets dealing with popular and electoral vote margins.
3) Many of the most successful players on PredictIt are excellent at playing the swings. Simply reacting to news quickly can net you hundreds (or thousands) in minutes, so there is often no need to hold for long periods of time.
Conclusion: It is certainly possible for people to know better and also make a substantial profit on PredictIt. The $850 limit does impose a limit on profits for a single bet in a single market, but the wide selection of markets on the website allow a dedicated individual to research and react their way to a profitable edge in virtually every market, yielding good returns despite the fees. There is a very good reason the website has a dedicated user base.