My guess is, the ash hasn't been analyzed to determine its origin, and when it finally is (probably years or even decades from now) it will be shown to NOT match Thera's.
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[snip] Tanr?ver said, adding that the ashes have been tested at the Geography Department of Ege University. Tanr?ver noted that the volcanic explosion in Santorini spread ashes in the region and triggered tsunamis throughout the Aegean, causing the Minoan civilization in Crete to collapse. “Once the ashes are examined, they will provide us the opportunity to see what changes the explosion caused in Smyrna and how it affected the people their culture. It will be an indicator like a touchstone. We will also be able to chronologically rank some events in Smyrna that for which could not set an exact date before,” he said. [/snip]
(note the near-complete lack of detail about the ash)
In 2012, some details about the excavations up to that time don’t mention the ash:
https://www.world-archaeology.com/features/smyrna-5/
In 2015, ditto:
https://archaeologynewsnetwork.blogspot.com/2015/01/smyrna-agora-excavation-perimeter.html
In 2016, still nothing about the ash, but assurances that the site would soon be open to tourists. It is again mentioned that the latest finds are from classical Greek times, which by the conventional dating scheme is up to 900 years after the supposed supereruption of Thera:
http://www.hurriyetdailynews.com/ancient-smyrna-to-complete-ring-of-history-105702