Even if they had really shitty agreements I bet they'll see something substantial. Reports are saying it was a 5.3 billion purchase price. That's a lot of money for 400 employees.
Having 400 employees doesn't say much about how equity is distributed, though. E.g., (made up numbers, I know nothing about Plaid's comp specifically) if an employee gets 1 basis point over 4 years, that's 0.01% * $5.3 billion / 4 = $132,500, which combined with a startup salary might land you at around Google L4.
The early employees will get millions of dollars. This is 1)an unreasonably large amount of money for anybody to make or 2)nothing compared to the billions that the founders and venture investors will make.