According to the text Current French; St. John, Jones, Stickland; Copp Clark Publishing, 1950s or 1960s? :
4.10 The indefinite and partitive articles are replaced by "de" before a noun used as the direct object of a verb when a negation implies the absence of what the noun represents. (Note that this replacement does not apply for "ne ... que".)
In other words, after "ne... que", indefinite and partitive articles do not change to "de" unlike the change that happens after other negative adverbs.
That rule suggests why "ne ... que" is sometimes described as a restriction rather than a negation.
This is one of a number of situations where what is natural in French is not natural in English. Yes, the French are used to eating little containers of yogurt. In English, the plural form is normally lost in translation and it does nothing to insist on "yogurts" here. That is why the singular (or "mass noun") form is accepted.