I'm surprised to see this downvoted. My understanding is that one can only contribute to a Roth IRA if their income is below a certain limit. For the 2021 tax year that is $139k. Plenty of founders are receiving salaries below that threshold in 2021, even after raising money.

If Thiel made this contribution for tax year 1998, then this would be before they even took any external funding. It's quite conceivable that he received an exceptionally low salary that year and relied on savings.