3/4 inch is the nominal diameter. The actual outside diameter is the same for all schedules so the same size threading tool can be used on all 3/4 inch pipe regardless of the wall thickness.

The table indicates the welder is qualified for 1 inch diameter (outside diameter) or larger. I take that to mean if the OD is 1 inch or larger the welder can weld it regardless of the pipe wall thickness (schedule) or the nominal diameter (nominal ID). That being the case, the welder is qualified to weld the nominal 3/4 inch pipe because the actual OD is 1 inch or larger.

Best regard - Al