Aramaic is not one language. It is a family with a documented history of more than two and a half thousand years, divided by period and by region, and Syriac is one member of it: the Aramaic of Edessa, in Osrhoene, which became the literary and liturgical language of a large body of Christian writing.
What separates one Aramaic dialect from another is rarely dramatic. It is a set of small regularities of exactly the kind nun provides. Syriac and the other eastern dialects form the third person of the imperfect verb with a prefix written by nun; the western dialects use a different one. A reader who knows this can often place a text from a single verb.
That kind of evidence matters because the scripts do not sort themselves neatly by language. Syriac letters were used to write Arabic, in Garshuni; a reversed pe belongs to Christian Palestinian Aramaic, a separate dialect that took the Syriac letters over for its own use; Sogdian and Persian writers added marked letters to the same set. Script and language part company constantly in this region, so the language has to be identified from the language.
The atlas keeps them apart for the same reason. Syriac the alphabet is one entry, Classical Syriac the language another, and a letter belongs to the alphabet. What the letter does in the grammar, as nun does here, belongs to the letter as a fact about how it is used.