I can see what you're saying, but in reality, companies do not trust ARM with their IP. They license ARM's IP, and then us that to create their own product, without ARM seeing their IP.
I'm sure that there must be some brand checking to make sure ARM IP is neither used outside of the license, or implemented in a way that would damage the ARM brand, but that is nothing compared to the view that TSMC get of the chips they're fabricating.
I do not fully know whether TSMC just take fully fledged designs, or actually assist in the layout and masking of the chips from the desired circuit diagrams, but whichever it is, they get to see the totality of a chip design. This puts them (and any other chip fabricator) in a position needing absolute trust.
The problem is not that ARM can't be trusted, it's that under Nvidia control, they could favor NV with reduced license costs, or even withholding some designs from others, giving NV a competitive advantage over other ARM licensees.
If NV had performance or costing advantages, then these are exactly the anti-competitive concerns that need to be investigated.