In every definition of "Non-Industrial" i've seen, it's always only been population of 6 (millions) or less. In T5 the definition is pop 4-6, in earlier versions it's been population 6 or less.
I think that the term "non-industrial" is mis-leading. A better term might be "moderate population".
Population and Industry are linked.
Given a typical workforce of 60% of the total population (we will assume that young children and their mothers are not being sent to work in factories), then a typical POP 3 world of 5000 people would have a workforce of only 3000 people. If HALF the workforce was employed in the Manufacturing sector (a staggering high percentage), that would only be 1500 workers to support all Industry on that world.
The Saturn Plant in Spring Hilll, Tennessee was the only factory to manufacure Saturn brand cars in the 1990's. It employed 3 shifts of 1000 employees per shift to manufacture 66 vehicles per year per employee.
The POP 3 world be an Industrial "Company Town" with only one factory employing 1/3 of the total planetary work force to produce a single product at 1/3 the factory capacity (1 shift) and the remaining 500 workers struggling to provide the plastic and steel needed to support the factory.
It takes POP to support Factories, and Factories to be an IND exporter world. If each world exports 1% of its maufacturing capacity ...
At POP 4 (50,000) and a more typical 20% of the workforce in Manufacturing, 6 thousand workers would support 2 factories (at 3 shifts) exporting 76 tons of goods per week.
At POP 5 (500,000) and a more typical 20% of the workforce in Manufacturing, 60 thousand workers would support 20 factories (at 3 shifts) exporting 762 tons of goods per week.
At POP 6 (5,000,000) and a more typical 20% of the workforce in Manufacturing, 600 thousand workers would support 200 factories (at 3 shifts) exporting 7,615 tons of goods per week.
At POP 7 (50,000,000) and a modern 10% of the workforce in Manufacturing, 3 million workers would support 1000 factories (at 3 shifts) exporting 38,077 tons of goods per week.
At POP 8 (500,000,000) and a modern 10% of the workforce in Manufacturing, 30 million workers would support 10,000 factories (at 3 shifts) exporting 380,769 tons of goods per week.
At POP 9 (5,000,000,000) and a reasonable high 30% of the workforce in Manufacturing, 900 million workers would support 300,000 factories (at 3 shifts) exporting 11,423,077 tons of goods per week.
At POP 10 (50,000,000,000) and a reasonable high 30% of the workforce in Manufacturing, 9 billion workers would support 3 million factories (at 3 shifts) exporting 114,230,769 tons of goods per week.
Changing the Percentage of the population in Manufacturing might half or double the goods per week, but POP has orders of magnitude impacts on the availability of manufactured goods and the number of factories.