I do understand what you're saying. However, the fact that AAPL shares are more liquid than AirBnB shares makes a difference to the way I think about valuation

I do understand what you're saying. However, the fact that AAPL shares are more liquid than AirBnB shares makes a difference to the way I think about valuations. APPL could be up or down tomorrow based on the example you just described but the same won't be true of AirBnB. If they IPO'd tomorrow, would they get a valuation of $1B? I don't know. I'm not disagreeing with the definition. I'm trying to point out that even though the maths might be the same, I see a difference between startup valuations (based on funding rounds) and market caps (shares * share-price) of listed companies.