To be clear, he is rich. But he's not crazy crazy rich, like nowhere near billionaire status.
With that in mind, his kernel is a key component of RedHat's, SuSE's and Canonical whole business, with at least two of those being multi billion dollar businesses.
His kernel is a key component of Android phones, which represent over 50 billion a year in hardware spend, and a bunch of software money on top of that.
His kernel is foundational to most hosting/cloud services with just mind blowing billions of revenue quarterly.
It's used in almost every embedded device on the planet, networking gear, set top boxes, thermostats, televisions, just nearly everything.
People with a fraction of that sort of relevance are billionaires several times over. A number of billionaires owe much of their success to him. Yet he is not among their numbers.
Now there's more to things than just a kernel to be sure, but across the hundreds of billions of dollars made while running Linux, there was probably plenty of room for him to carve out a few billion for himself were he that sort of person, but he cares about the work more than gaming the dollars. I have a great deal of respect for that.
Means that while he may not always be right, but I at least believe his assessments are sincere and not trying to drive some grift or cover some insecurity about being left behind.