In one exercise DL says ריבה is jam, in another exercise DL says ריבה is jelly, ריבה is jam, גילי is jelly
There is a difference in the US as well. Jelly is more of a juice form, and to my taste seems far more sweet. Jam (which I prefer) is more of a crushed fruit form, and has more of a full flavor of the fruit (again, my opinion on taste). Never the less, they are indeed different.
"There is no jelly" is poor English grammar. "No jelly" doesn't exist so it cannot be "there"!
Who said so? This is the classical germanic structure to deny existence. This is precisely what says this is a germanic language (among other things) and every single grammar of English I've seen, even formal ones, acknowledge this structure.
And language is not necessarily a mirror image of reality, but of the history of social perceptions about it. If grammar ought to obey logic, there would not be the word darkness (as the absence of light isn't anything to be), nor the word ship would be feminine (specially in a language that almost has no gender), nor any word about a non animate thing in any language, and a long etc. of nonsensical structures on the logical sense.
In conclusion, grammatical rules are independent of logical rules and none is useful for discussing about the other.