The article doesn't expound on it, but it very much depends on what Arabic means to you. Depending on the answer, it's really a dozen different languages. I know people who only speak their own darija and classical literature is utterly obscure to them.
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Sorry to disagree, but no, they are not dozens of different languages. The challenge with Arabic is that it has a rather large vocabulary, and different regions use slightly different vocabularies.
That being said, Darija, or rather North African Arabic is a messy mix of Arabic and Tamazight. Which can be difficult for Middle East Arabic speakers to understand.
For reference, I speak Darija and understand both classical and modern Arabic. It would take me a few days to adapt my speech to other regional variations of arabic.