Comment by antonvs

The point is that in saying "I bought a car", you're not attempting to specify which specific car you bought. The fact that you obviously did buy a specific car isn't relevant to the grammar of this.

If the audience already has some context about which specific car you were going to buy, then you could say "I bought the car yesterday." In that case you're referring to a specific car that the audience is expected to already be familiar with.

I think some of the confusion over this seems to be coming from attempting to apply knowledge that you have to a sentence when the audience wouldn't have that knowledge.