Does adding sugar to tomato type sauces affect acidity?
Weak organic acids such as those found in fruits and vegetables (citric acid, malic acid, tartaric acid) don't react with sugars. 1 There is no change in acidity, which you correctly defined as measured by the pH. 

At the same time, sweet and sour are two tastes which are real antagonists - adding something sweet actually reducess the sourness we perceive, as opposed to just distracting us from it. So the sugar changes the sourness (the taste), but not the acidity (the chemical property). Hearing it otherwise comes from the fact that most people don't even realize that there is a difference between the two terms, and use them interchangeably. 



1 They can partake in reactions as catalysts, for example to create invert sugar when cooking up sugar syrup, but the acid itself does not react away.