But somehow, it is his company that he brought to that point, and also why would he care for being spied on by a country or another? There is nothing that said that he had to swear fidelity to the US.

Few companies can afford to be disloyal to their government for any reasonable amount of time. Not easy to see in the West because most government are quite closely lobbied, but it's important to be aware of. Nvidia is of strategic importance and has been part of the USA's sanctions regime against its so-called near-peer adversaries for quite some time.