I'm just trying to figure out this exact thing.
I tried signing into Microsoft Store on PC using the same Microsoft account as on the XBOX. It signs in and syncs data when it starts, but there are no save games to load once Stardew Valley starts up.
There might be some incompatibility between the XBOX version of the game and the PC version which makes it hard to synchronise the save games between devices. Only Eric can confirm.
My next step would be to see if I can find the save games cached locally on the XBOX, but I think I will need to enable developer mode to do this.
Finally, it is likely that the files are encrypted. Maybe accessing the file system of the XBOX while in developer mode will decrypt them

Only way to find out is to give it a go.