You just call it Arabic because Arabic is used for "high" functions in the region; If we were diglossic Levantine/Aramaic instead of Levantine/Arabic you would say the same.
Less than 90 km from NNT's hometown is a village where they do in fact still speak Aramaic, while of course still being diglossic in Arabic: Maaloula, in Syria. Despite heavy Arabic influence, this village's language has never once been mistaken for Arabic; its own people call it siryêni, and European Semitists recognised it as Aramaic as soon as even simple wordlists became available. If you happen to be Levantine, try listening to some of it (eg here) - how much of that do you understand? The same is true of other relict Semitic languages within the Arabic-speaking world, such as Mehri or Jibbali or Soqotri or Neo-Mandaic. I have more than one book in which Soqotri or Jibbali speakers attempt to prove that their languages are really Arabic, for much the same reasons that NNT wants his language not to be Arabic - but, notwithstanding the speakers' desires, Semitists had no trouble proving that these languages were not descended from Arabic. Conversely, the "high" languages of Malta have always been English and Italian, yet, despite Maltese nationalists' best efforts to show that Maltese was really Punic, European Semitists had no difficulty in identifying it as descended from Arabic. So, no, Semitic historical linguists do not base their decisions on what kind of diglossia happens to be around, nor were all those 19th-century German Orientalists secret agents sent back in time by the Baath Party. To the contrary, almost all Semitists I've known would be far more excited to discover that some undocumented variety was a new Semitic language than to find out that it was "just" another dialect of Arabic.
How do linguists know that Spanish is descended directly from Latin, not from Italian? Simple: we look for cases in which Italian has made a change - innovated - and Spanish hasn't. Such cases are easy to find: for example, in Italian original *fl has become *fi (thus fiore "flower") and original long *e in open syllables has become i (thus di "of"), whereas in Spanish original *fl remains fl, and *e e (thus flor, de). If Spanish were descended from Italian, then these changes would all have had to have happened and then reversed themselves in Spain, which is very unlikely. We can know which form was original not just because in this case we have copious ancient data, but also by using comparative-historical reconstruction. The full toolkit would take too long to explain here (my favourite textbook is Lyle Campbell's Historical Linguistics), but basically, we:
- establish sets of sounds corresponding systematically to one another;
- figure out whether these correspondence sets systematically occur only in certain environments, and, if so, see whether there are any other correspondence sets occurring only in non-overlapping environments that they can be unified with.
Now, let us apply this to Levantine, Arabic, and Aramaic. Reconstructing the common ancestor of Aramaic and Arabic (see eg here or even just here) shows that Aramaic features a number of innovations not shared with Arabic; conveniently, many of these are mergers. In particular, in Aramaic *`, *ʁ (gh), and *ɬʼ (lh) all merge to ` (ayin); *x (kh) and *ħ merge to ħ (heth); initial *w and *y merge to *y. In Arabic, all of these distinctions are maintained. Now, the nice thing about mergers is that they can't be reversed; once two formerly distinct word classes feature the same phoneme, there's no way for the ordinary speaker to recover the distinction. A monolingual Aramaic speaker has no way of telling that the ` in 'ar`ā "earth" (< *'arɬʼ- + -ā) used to be pronounced differently from the ` in ṭar`ā "door", or in `aynā "eye". In Levantine, all of these distinctions are normally maintained, just as they are in Arabic; أرض has none of the consonants of عين. QED. (In fact, historical linguists have also succeeded in identifying some Aramaic loans into Levantine Arabic by finding the small minority of words in which these distinctions were lost.) In fact, you don't even need to look at phonology to figure this out; the grammar provides plenty of clues. In Aramaic, for example, almost every noun ends in -ā, except in a few specific contexts. This is an innovation specific to Aramaic, accomplished by gluing a former demonstrative on to the end of the noun, and preserved in every modern spoken Aramaic variety. In Arabic, it never happened - nor, obviously, in Levantine.
Of course, NNT shows no signs of even being aware of the relevance of regular sound correspondences, mergers, or any of the other elements in a historical linguist's toolkit, much less of accepting them as definitive criteria for language classification. At one point, however, he vaguely expresses the criterion he thinks should be definitive:
Now that we've seen a little bit of how linguists determine what comes from Arabic and what comes from Aramaic, we're ready to look at the results of this criterion in the next post. You should be able to guess the answer already...