(a) that they build robotaxis ~5 years ahead of competitors
(b) they continue to sell ~50% more cars each year (they have a stated goal of 20m cars/yr by 2030)
How much is a robotaxi worth? A regular taxi can drive ~100k miles/yr @ ~$2/mile. Let's say that Tesla robotaxis undercut on price by a factor of two, thus making ~$100k/yr in revenue. Let's say 50k goes to the buyer, and 50k goes to Tesla. Five years of that would be worth $250,000. Let's divide by two to be a tad more conservative. If you're selling 20m cars a year, that's $125,000*20m = 5T of value in cars in a single year. If Tesla were doing that in a single year, their market cap would be in excess of $10T]. If you think Tesla has a 1 in 10 chance of pulling off the above, then you'd expect a fair market valuation for them today of 1T — which is roughly what the market puts them at.
Plus add Tesla Energy, which could be as big as the car business.
Plus add Tesla Bots (labor), which is probably the largest market in the world.