Problem is Google will never have high quality content from big publisher, it directly compete with their revenue so I'm not sure how they're going to get game on this platform.
Apparently it's a Goldman Sachs estimate. The last time that number was explicitly listed (in court filings) was $1B in 2014. The more apt metric would be what YouTube pays for original content, though I've only seen estimates/projections (a "few hundred million"):
https://www.engadget.com/2018/02/23/youtube-red-original-pro...