Whether ancient Macedonian was a dialect of Greek or a distinct language related to it has been argued for well over a century, and for most of that time the argument was conducted almost without evidence. The Macedonian court wrote in Attic Greek. Macedonian inscriptions are in Greek. What the population actually spoke survived mainly in a scatter of glosses recorded by later lexicographers and in personal names, neither of which is a satisfactory basis for classifying a language.
The Pella tablet changed the evidential position because it is a continuous text, of some length, written by an ordinary person rather than a chancery, in a Northwest Greek dialect carrying features that have been read as Macedonian. For one side of the argument that settles the matter: what the tablet shows is a speaker of a Greek dialect, and the writer was not a courtier performing Attic.
The atlas records the linguistic claim as debated rather than attested, for a reason that is methodological rather than nationalist. One document written by one person establishes what that person wrote. It does not by itself establish what the population around her spoke, and the possibility that a Greek dialect was in written use alongside a different spoken vernacular is not excluded by a written text. The tablet is the best evidence there is, it points in one direction, and it is still one tablet. The dispute is also entangled with modern political claims over the name Macedonia, and that entanglement is part of why the scholarship on it is worth reading carefully.