David Grusch is literally talking heresay with zero first hand accounts. Heresay is not allowed in courts for a reason. Snowden blew the lid off an international conspiracy, and he had gigabytes of receipts.

Grusch has talked to people who work in information constrained spaces working on advanced materials with fancy codenames. If he was able to give literally any proof besides words coming out of his mouth and "trust me bro, they could be interdimensional", I would give him some credence, but there is nothing.