Intel foundry is for all US... actually all global companies who don't want TSMC to hold monopoly power on wafer pricing for edge nodes. If it merges with AMD, it will lose that appeal. Certainly It would absolutely make sense for Intel to split into product and foundry and the product side merge with AMD, it would create massive value for stock holders but that's not the intent of the merger.
No, AMD won't buy Intel foundry. The Gov't needs to unlock the chips act and subsidize Intel foundry until it can sustain itself and at the same time work to help global companies give orders to Intel ... a small portion of their orders to help Intel foundry develop and catch up.
TSMC was started with heavy subsidies from their Gov't, otherwise it would not have been possible to survive.