Here are some counterarguments:
There can be scenario’s where the agent cannot change his source code without processing observations. e.g. the agent may need to reprogram himself via some external device.
The agent may not be aware that there are multiple copies of him.
It seems that for many plausible agent designs, it would require a significant change in the architecture to change his utility function. E.g. if two human sociopaths would want to change their utility function into a weighted average of the two, they couldn’t do so without significantly changing their brain architecture. A TDT agent could do this, but I think it is not prudent to assume that all actually future existing AGI’s we will deal with will be TDT’s (in fact, most likely most of them won’t be it seems to me).
So I don’t think your comment invalidates the relevance of the point made by the poster.
Yeah, I was talking mostly about idealized UDT agents, not humans.