

This is not a proper talk by meta that you could just “hear them out”. They explicitly said off the record and confidential, there’s no reason for that if it’s something innocuous.
They plan on showing demos of their product to them or talking about potential features it might have. Boom, they require an NDA.
I don’t think you understand how the professional world works or how common NDAs are. I’ve signed NDAs while going through interview processes at FAANG and other large companies just so that we can talk freely about projects I might work on. Especially for a company like Facebook where everything they do will get about a dozen news articles written, they’re going to make you sign an NDA for any conversation about an unreleased product.
That’s standard practice if you’re going to be talking about an unreleased product.