I don’t think salt modified the acidity, though. What they do is activate the salty receptors which share nerve pathways with bitter receptors, so you detect less of the bitterness in the coffee.
Edit: I can’t find any reputable sources for the sharing nerve pathways part, but a couple of paper abstracts I found do show that saltiness suppresses our ability to detect bitterness.
In the same paper abstracts, they also mention how bitterness itself suppresses sweetness, so by suppressing bitterness with salt, you also make any sweet notes in the coffee stand out better. This might be another reason why salt makes it more palatable.