“I am still skeptical of this psak (without an actual teshuvah with sources)”
So am I. I ran this past one or two learned people of my acquaintance and they didn’t believe it was true. One of them showed me an article by Rav Aryeh Kaplan zatsal about how there are opinions that the world is more than 6 thousand years old.
And on a previous thread, I posted a quote from Rav Hershel Schacter stating that this is not true.
In any case, I don’t think it’s right for someone to make a statement like this (which is in effect Motzi Shem Ra/Loshon Hara on a large number of Jews) without a source. Telling people to call Rav Eisenstein does not qualify as stating a source.
Without a source we are not allowed to believe you and therefore, I believe it’s assur for you to make this statement.