That’s mostly a reductio ad absurdism, but even Plato didn’t believe that people had the potential to be ethical objectivists, and he certainly wasn’t a postmodernist.
Acting as an authority on which instances of political speech are true and which are false may be easier in some cases than others, but there’s no clear place to draw that line, and there’s no way to avoid imposing your own opinions on others.
You don’t need perfection. As long as you look for obvious objectively false statements you will reject quite a bit of propaganda / disinformation etc.
You pretty much do in this case, since propaganda writers will adapt around your filter. Furthermore you have the problem of determining objective fact to filter on, which is a massive problem itself, as can be seen from the need for a filter in the first place.
Acting as an authority on which instances of political speech are true and which are false may be easier in some cases than others, but there’s no clear place to draw that line, and there’s no way to avoid imposing your own opinions on others.