TD: Very good observation of yours. No one will argue that Modern Greek and Koine Greek are two thousand years apart with great differences. However, as you noted, many modern Greek folks do have familiarity with ancient texts. And Greek has changed less than English has through the centuries. An educated Greek can follow quite a bit of Koine Greek even without formal training.
See Paul L. Kaufman's comment on this on page one for those who missed it. Here's another one from Wikipedia: "Ancient Greek texts, especially from Biblical Koine onwards, are thus relatively easy to understand for educated modern speakers."
And as Earnest indicated, it is not unusual for the WT Society (who have access to multiple resources) have someone specializing in both Koine and Modern Greek available to assist them in the translation process. Earnest said: "On the other hand, I do know of a brother in Greece who is equally at home in both ancient and modern Greek and who has been assisting the Watchtower Society in translation for the last 30 years so..."
This is not surprising because to produce the NWT Greek edition required perhaps a bit more than other versions into other languages. The WT took seven (7) years to produce their modern Greek version.