Psionics is a type of spellcasting in D&D because there's no Psionicist Class. Because there is no current framework to present psionic sciences and disciplines.
If a Psionics book were to be released, all the "Psionics is Spellcasting" stuff would be retconned because they'd have the powers and functions. Just like it was in 2e and 3e.
Though if you pay attention to what 5e's designers say: Psionics -can- create spell effects that are subject to things like antimagic zones... but that's not -all- Psionics are. Because while they can create spell-like functions, the rest of what they do doesn't get stopped.
Some people refuse to understand these facts and chooses to believe, wholeheartedly, that Psionics is just magic and the Weave is a perfect design choice rather than a kludge to describe supernatural power.
And you know? Yeah. I'd say that Athas had a Weave, back when the sun was blue and oceans churned and plants bloomed green over most of the world. Athas had a weave when the Pristine Tower was built and burned away the Brown Tide, turning the Sun Yellow. Athas had a weave Rajaat gathered his fifteen. Athas had a weave when the genocidal Cleansing Wars began. But that Weave was defiled, destroyed, and left magic in Athas with tattered remnants of what once was.
Which Rajaat's followers and other Defilers tear apart with the casting of every spell.
But it should also be noted: Athas had Psionics -first-. It was only when the Pyreen formed, Rajaat among them, that Magic was discovered. The Age of Magic came after the Green Age. And it was his magic that made the sun turn from Yellow to Red.
At least... That's the story -I'd- tell, if I were the writers. But they'll probably go for the easier one:
See... Dark Sun is cut off from the rest of the Multiverse. Instead of all kinds of Transitive Planes and Outer Planes and so on and so forth, it only touches two surrounded by a third. The Grey and the Black within the Hollow. The Hollow is oblivion. It is nothingness. And it separates Athas from the rest of the Multiverse. Souls don't go to Arcadia or the Abyss or anywhere fun like that when they die. Spelljammers who try to get to Athas either get destroyed in the Hollow or miraculously make it through and become trapped.