It works better if Biden ultimately gets replaced, but the purpose here is stated in the opening statement: "these two candidates, they are both similarly challenged".

Then he gives examples of Biden being senile and says that Trump is old. Notice by the end you're tempted to think "yeah they are both old, they're the same!"

You see how advanced this propaganda is compared to say John Oliver; nobody watches Oliver's machine-gun you with bullshit until you nod and doesn't know they're being programmed, but people watch Leibowitz's bait and switch and don't even realize he took the senile label off Biden - started out the clip senile and ended it just old - and slapped it on Trump's back.

This effect isn't an accident of comedy, both the Johns do it on purpose. Leibowitz knows that Biden is senile and Trump is not, and he purposely set out to conflate them in the audience's mind because he hates Trump.