I think they're using some kind of "daily equivalent average pay, factoring in exponential growth of the stock divided by actual days worked over a career" -

> Under the modern tech landscape, stable “hyperscale ultra-growth” companies are paying experienced employees the equivalent of $10,000 to $50,000 per day if we include the value of their exponentially growing yearly stock grants.

Assuming a $250k salary, that's only about $1000/day. But if you're able to bank $50,000,000 in stock grants over a 40 years career (invest early and often in a high-growth company), that averages out to $5,000 per day.

Kinda dodgy math, should been better clarified, and that's still somewhat ambitious; but I think that's the idea behind it based on a couple allusions throughout the article.