Sorry, not much to back it up other than lack of knowledge of any shared-wafer fabs in the US. From what I can tell, having a fab in the US is more a consequence of wanting to make one's own chips, not 'chips as a commodity' (foundry) business:

https://en.wikipedia.org/wiki/Foundry_model

Additionally there is this: https://en.wikipedia.org/wiki/List_of_semiconductor_fabricat...

Note as a result of this comment I did some digging and it does seem like Global Foundries does do wafer-share: https://www.globalfoundries.com/design-services/multi-projec...