Yes, that's another question, why does the IPC requires that "The fixture drain for the standpipe serving an automatic clothes washer shall connect to a 3-inch (76 mm) or larger diameter fixture branch or stack"? The UPC has no such requirement, so it's a bit foreign to me.
Regardless, if the stack in the photo in the OP has no drainage coming down from above (which had best be the case, as otherwise the standpipe is not properly vented), then the stack could be 2", and the 2" size could be maintained until the standpipe drain joins another fixture. The definition of "fixture drain" is a drain carrying only one fixture, and the definition of "fixture branch" is a branch carrying at least two fixtures. So as long as the fixture branch is at least 3", the requirement in 406.2 is satisfied, and there's no need for the stack to be 3".
Cheers, Wayne