For example, an arrogant "10x" developer who doesn't team-play and work 9-5 for work life balance may be the "best" when the company has grown to a large size, but for an early stage startup, this can act as the #1 reason for the demise of the startup because it messes up the morale and culture.
As soon as a startup becomes successful they have hired an attractive anthopology gradute to be "head of people ops" etc. After this it is mostly luxury ideals and gimmicks.
At one time every startup claimed to only hire the best. Then they mostly failed. They are still out, being the best, that is currently avialable, in a commutable distance, at below market rate.
Plus why would you want to work for a startup? That is the most unstable stage of a company. A large portion of startups fail. I've done it several times. So you are better off consulting a startup than working for one.
Startups will give you a great marketing speach and try to promise if you work for cheap or free that you get xxx amount of company stock and you too will get rich = STARTUP
If you are the best at what you do start a consulting company. That is what I am doing right now.
But "best" is a subjective term. In a job post referring to "best engineers," I think it's a cop-out and just fluff.
Extremely talented people will gravitate towards those companies and you'll find yourself surrounded by raw passion.