All evidence points to Plato making Atlantis up for rhetorical purposes. Everybody points back to him.
Yep, yep, yep! Plato is the singular author of that allegory, and Aristotle confirms it.
On thing I'd like to add:
Only 13 years before Plato published the Timaeus (c. 360 BCE) while living in Athens, the city of Helike literally became "submerged" in the ocean due to a rare seismic event (c. 373).
If Plato didn't witness it with his own eyes, he almost certainly would have felt the seismic shock, followed shortly thereafter by the news of a horrific event just miles East of him. And if he didn't feel the shock, he definitely would have spent the next few months discussing the narrative of a known, friendly city that sank into the sea, (so long as that event captured public interest).
I maintain Helike was the inspiration behind the fate Plato assigns to his allegorical city.