Here ya' go. I dug around and found a discussion on that rule. I'm not sure when it went into effect, but I think it came out at least a couple years ago.

9-15 Pit Procedures During Race

The fuel handler must be in control of the fuel can at all times when fuel is being added to the car. The fuel handler will not be permitted to perform any adjustments or other pit stop procedures while the fuel can coupler is engaged with the car-mounted adapter.

Matt Clark explains it like this. “This is a rule to help with safety in my opinion. I think Nascar is wanting Gas Men to concentrate on gassing the car and nothing else. They don’t want a gas guy trying to do too much. Gassing is a dangerous position and making it as safe for all the crew guys is a priority in my opinion.”

So how will this effect teams doing their pit stops?

“Some teams would have their gas guy stick the wrench in the left side wedge hole while gassing. Now they can’t do that.”

So can gas guys help out during the stop now or are they limited to just gassing?

“My interpretation of the rule above is that they can. As long as they’re not plugged into the car they can do whatever they want to help stops. I think the rule just wants to make sure when they are gassing that they aren’t performing any other jobs.”
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I'm assuming if he helped the tire changer, he must have already had the filler disengaged.