I couldn't find what he was talking but found this interesting pdf that's more or less related to the topic. Clearly something close to what he's talking about is being considered and researched, at least as early as 2005. https://www.rand.org/content/dam/rand/pubs/technical_reports...

I think it's been raised in the context of a fair few sightings where there's supposedly been a very large craft moving silently and relatively slowly, e.g. the 2000 Illinois sightings (referenced in a great song by Sufjan Stevens). I was looking into this one a few years ago and found references to a private company testing blimp platforms for military purposes around that period, although I can't find it off the top of my head.

More recently, the object spotted hovering off Hawaii last year that resulted in fighters scrambling was also proposed to be a modern balloon-based drone, of which there are a few currently being developed.

Edit: here's a source arguing that the Illinois sighting was a regular advertising blimp

https://skeptoid.com/episodes/4435

Emphasis on the "rumor and speculation" aspect but it's been chattered about for many years:

Stealth Blimp: https://duckduckgo.com/?t=ffcm&q=Stealth+Blimp&atb=v269-1&ia...