Depends where it was - Spain: fuelling while boarding prohibited unless airport fire service in attendance; UK: no problem, but there are certain procedures which need to be followed simliar to those you mention (i.e. seatbelts off, rear slide area clear etc etc). In any case, specific company procedures will apply but must abide by local regulations.
Just because the ground staff told you that delayed boarding was due to refuelling, doesn't mean that was true - more likely the crew were giving the wheelchair-using passengers time to get settled properly before filling the cabin with a scrum of seat-grabbing pax.