Archeological Excavations: The Discovery of Evidence
Sampson's credentials appear to be solid enough:
"One of the most important excavations’ locations of recent years, under the responsibility of the inspector of antiquities Mr Adamantios Sampson, is the island of Gioura in the Sporades Islands’ complex. . .Adamantios Sampson is famous for his research on the prehistory of the Dodecannese (excavations on the island of Gyali in Nisyros - a source of 'obsidian'; study titled 'The Neolithic Period in the Dodecannese'), of Evoia (Cave of Skoteini Tharounia), the very important excavation in Manika of Chalkida, the prehistoric research in caves of Achaia and Kopaida e.t.c. Today he is the supervisor of the Inspectorate for Prehistoric and Classical Antiquities of the Cyclades."
So that much seemed to check out. The next thing I did was look more closely at the find in question and see if it was open to other interpretations besides the one the article advances. If you click on the link I gave for the article (Greek alphabet was in use at 6000 BC) there's a picture of the potsherd being discussed. IMO questions could be raised both about the interpretation of the symbols on the potsherd and about the proposed dating. There are characters that could be interpreted as Alpha, Upsilon, and Delta as the article proposes, but I could see what they're interpreting as Upsilon also being interpreted as a lower-case Gamma, and I could furthermore see the whole set of symbols being interpreted as pictures rather than symbols, with the alleged Upsilon/Gamma being branches of a tree or something (in the picture the Upsilon seems to be "growing" out of a symbol below it). I'm also not confident in the dating of the potsherd, as I know there are problems with radiocarbon date skewing in the Aegean due to the Thera eruption (see Prehistoric Archaeology of the Aegean: Chronology and Terminology:
"NOTE: In general, absolute dates for the Aegean Stone and Bronze Ages are not yet very reliable and many different sets of dates are often in use for one and the same phase or period. A major debate has been raging since 1987 over the absolute date of the great volcanic explosion of the island of Thera/Santorini early in the Late Bronze Age. As a result, absolute dates within the first two-thirds of the second millennium B.C. (ca. 2000-1350 B.C.) are presently in an unusually active state of flux. It is therefore always best to describe an archaeological assemblage in terms of a relative chronological label (e.g. Early Helladic II, Late Minoan IA, etc.) rather than in terms of its supposed duration in calendar years B.C.").
So I could see some room for interpretation there with regards to both the meaning of the symbols and the dating of the potsherd. I'd personally find it more parsimonious to state that this find may show that certain symbols later used in the Greek alphabet were in use at 6000 BC than to state that this find proves the entire Greek alphabet was in use at that early date. It does raise questions about the relation between the Greek and Phoenician alphabets; however, I could see ways of interpreting the find that would be consistent with what Herodotus says (e.g., there was contact between Greece and the Middle East prior to the Phoenician period, so there may have been prior exchange of certain symbols which preceded the final transmission of the Phoenician alphabet to Greece mentioned by Herodotus). As a rule of thumb when Herodotus conflicts with modern historians, I tend to assume that Herodotus should be given the benefit of the doubt, since he was so much closer to the events than we are and had access to sources we no longer have. Still, that said, it's an interesting find that warrants serious attention.
Besides the testimony of sources like Herodotus, we have early examples of Greek alphabetic writing and examples of Phoenician writing, and it's beyond question that the Greek alphabet is derived from Phoenician (or some closely related system).
The Hebrew alphabet was similar--for the Hebrew letters see Psalm 119 which has 8 verses for each letter (aleph, beth, gimel, daleth, he, vau, zain, cheth, etc.). The present-day Hebrew letters are written a bit differently, since they use the Aramaic letters. The oldest Greek letters are closer to the Phoenician letters in appearance, and were written from right to left. The Greek letter names are clearly derived from the Phoenician names, and the order of letters is almost the same (the Greeks made some minor changes, and created a few new letters).
Greek is one of the so-called Indo-European languages; the ancestral language (which English, Latin, Armenian, Celtic, Persian, Russian, Hindi, and many other languages are ultimately descended from) is called Proto-Indo-European (or PIE). The date when PIE was spoken is not exactly certain, but was probably later than 6000 B.C.