There are around 5000 active kernel devs, they are generally highly skilled and therefore highly paid, and they've been working for a lot longer than 10 years.
So doesn't seem that unlikely based on your estimates.
Linux kernel has been in development since the nineties, not just for the last ten years. Also 5000 contributors is a lot more than 2000 from gp's comment.
Let's ignore the years before dotcom boom since the dev community was probably much smaller, and assume an average of 3500 contributors since.
That's 25 years * 3500 contributors on average * 200k salary (total employee cost, not take home) = $17.5b
So doesn't seem that unlikely based on your estimates.