Setting that aside, what is knowable is one’s actions. If someone contributes to organizations and initiatives, that is sufficient to show material support. Especially so at the financial scale at which Theil provides support.
I’m not interested in whether or not someone is “truly” an X or even if someone “truly believes” X. It’s enough for me to know that someone provides material support for X.
Finally, you didn’t define what “greater nationalism” is supposed to mean, nor is it rigorously defined. In any case, it’s a pretty high bar for anyone, even a billionaire. Petty nationalism is bad enough.