I've always interpreted this sort of valuation as more about "potential best-case acquisition price" than about real market cap. Instagram was "worth" a billion dollars, because Facebook said so; that doesn't mean it had a $1bn market cap. But when VCs talk about you having a "$100MM company", that's what they mostly mean: that there might be a Facebook or two around who would pay that.
I'm actually curious, since we've seen a few (not many, but a few) tech IPOs of these SV darlings in the last five years: how well does their publicly-traded market cap today align with the projected valuations VCs invested at?
Your average entrepreneur, say Bob from Bob's Plumbing, would be really happy to have his company build to $10MM over a decade or two.