That figure is based off of a recent internal valuation that was made public because it kind of had to be, because Twitter now pays employees partly in equity (I'd rather take monopoly money, but I digress lol).

So basically that's the number Elon himself admits the company is worth today - some people say less, but honestly it's impossible to pin an exact number on these things unless someone is actually offering to buy it (or if it's public and you can just look at the market cap)