That is the better question. Unfortunately the revenues of startups are usually trade secrets. Funding events are based on investors looking at revenues, so they're the best indication available.

First off, that data isn't (and will never be) available for new-ish startups.

Young, high growth startups often have irregular revenue and no profit. The early years of Facebook would've shown weak revenue and no profit. Most smart people knew that it had a disproportionate shot at big returns long before it had meaningful revenue.

It's probably a pretty good proxy. The charts in the article are dominated by a few huge investment rounds that resemble privately offered IPOs. If you think going public would be a good indicator of success, that's closer to what's going on in those charts than it appears at first.