The two party duopoly selected Biden, so Trump would win again in 2020.

I have no illusions about the actual nature of US "democracy".

Having said this, the video is so bad that it is probably intentional.
Biden even forgot what he had been discussing in his confusion (his family).
This would certainly be anticipated, and planned for accordingly.

These presidential campaigns are multi-multi-multi-million dollar operations.
I seriously doubt the campaign would insert trivial distractions like this by accident.
Biden isn't going to be allowed to improvise (with props, etc.). Each interview would be rehearsed (by someone if not Biden).
They know about filming reflections, etc.

This is probably intentional, because the Democrats hate Trump (in the most literal sense).
I doubt many Republicans actually hate Biden (it's pity), but they can help but notice he's fully senile.

The democratic party had numerous other " party candidates" to select.

It has to be deliberate.

I didn't check to see if it was a live stream.

If it was not live, then it's safe to assume it was deliberate. They could have edited it out.

We should also assume that the Democratic voters will never see this video on the news that they watch.