There are also cultures that aren't very diverse that are pretty high-trust: Denmark, Sweden, Norway, Japan, even China.

Which goes to say, diversity likely has very little to do with whether a society is low-trust or high-trust. It's more about politics and policy.

This doesn't seem like a conclusion that's supported by the available evidence.

We have examples of homogeneous cultures that are high trust, and ones that are low trust.

We have examples of diverse cultures that are low trust, but none that I'm aware of that maintain high trust over time.

The best fitting hypothesis would be that homogeneity is necessary but not in itself sufficient for a high trust culture to be built.