The definition of multimillionaire is a bit vague here, but I would assume the OP was referring to being in deca-millionaire territory. That is much more rare to accomplish at a FAANG company.
The average SWE at FAANG can become a deca-millionaire over the course of 20-25 working years.
Starting from $0, this requires saving $125k a year for 21 years to reach $10.06M at an 11.88% interest rate, which is the S&P500 average. Well within the capability of any senior SWE @ FAANG, and it only takes 4 years to get to senior.
But then capital gains taxes hit.
Anyways, yes, a startup employee on the other hand can become a deca-millionaire in a year or two.
Crush it at FAANG for a few years and don’t spend like crazy and you’ll be a multimillionaire.