..in general Greek speakers (whether they are JW or not) cannot deal with ancient Greek at all.
I would be careful about saying that in a roomful of native Greek speakers The ancient dialects are often taught (starting) at the middle school level and are still used liturgically in the church. Many Greek speakers have more than a passing familiarity with them.
Consider old English as an counter-example. Something like this might not be entirely intelligible today:
Oft Scyld the Scefing from squadroned foes, from many a tribe, the mead-bench tore, awing the earls.
..but an English speaker is still miles ahead of a non-English speaker when it comes to the language in its ancient forms.