Can a site with no revenue still be valuable?

Yes, and the standard valuation approach prices it at zero, which is where the opportunity is.

A property being crawled heavily, ranking on the second page for terms with real commercial intent, and earning nothing today has a materially different future from a property earning the same nothing with no visibility at all. A profit multiple cannot tell them apart.

The caveat is that "valuable" and "sellable" are different words. A property with no revenue is worth something to a buyer who can supply the monetization. It is worth very little to one who cannot, and that narrows the market considerably.