If Trump had actually told "whoever he needed to tell" about the declassification, then there would be a record of it being declassified. That's how classified information works. The president decides to declassify something, he tells his staff what the information is to be declassified, they send out a memo stating the information being declassified as well as the date, time and reason for the declassification, and all parties holding the information are notified and the information is declassified.
Since there is no record of the declassification, Trump didn't actually declassify anything. Him simply saying he did it without any record of it happening isn't declassifying something.