Plato may have had actual historical data about Atlantis. If so, it has been lost, along with much of the other information passed on from earlier times. A great many ancient records are said to have burned in the great library in Alexandria during Julius Caesar's fight with the Egyptians.

If Plato was serious about Atlantis (and one assumes he probably was), it seems pretty evident that he did not mean (the remains of) Thera or Crete. They were definitely not located "beyond the pillars of Hercules", but were within easy reach of Greek ships in Plato's time.

Why would he say "beyond the pillars of Hercules" unless he meant it? After all, it is a rather striking thing for him to have said. Beyond the pillars of Hercules was a place where Greek vessels did not willingly venture...perilous and unknown waters, in their opinion. Their galleys were not suitable ships for venturing into the deep Atlantic.