I thought the main reason is that AMD still needs to use GF's manufacturing capacity (according to their Wafer Supply Agreement) and it also makes the chiplet design cheaper (the high-performance node is only used where it really matters).

That could also be true. I remember from an adoredtv video it was mentioned that IO transistors are much harder to scale down. The guy certainly seems to do his research and know his stuff, but I suppose at the end of the day it is a youtube video. I have not checked into this myself, just taking adoredtv's word at face value.

I think that was poetic license on his part. Mobile SoCs and modems are scaling quite well on 7nm. The IO chip is already low enough power for desktop/server applications and is small enough for good yield. 7nm would just be more cost for no gain. And using 14nm allows AMD to meet the WSA. Otherwise they probably would have done a TSMC 16nm IO die and kept their entire supply chain in Asia.