Why do people say that cooking in a cast-iron pan will build the seasoning, while also saying that you should never cook to the smoking point?
Yes, the two pieces of advice are contradictory. You have to choose which one you prefer to follow. If you decide to only use your pan below the smoke temperature of oil, the seasoning will not build up during cooking.
In typical use, people do heat their pans above the smoking point, and the seasoning does build up. This is how cooking has been done for centuries, with tasty results, while cooking without heating the oil up produces not-very-exciting results for certain foods. People can choose to use low-heat cooking methods and avoid the &quot;smoke point&quot; part too, but that is not what usually happens. There are many factors why more people use the &quot;high heat that seasons&quot; approach:

They have never heard the &quot;smoking point&quot; advice and use their cast iron pans the usual way
They are not aware that they are heating it above the smoke point
They know the &quot;smoking point&quot; advice and choose to disregard it and cook the usual way
They would like to follow the &quot;smoking point&quot; advice but it is physically very difficult  to do it with a cast iron pan
(Kinda combination of the last two) They are aware of the difficulties of remaining below the smoking point in a cast iron pan, and choose to use a different pan when they want to stay below the smoking point.

Because of the last one, I would suggest that, if you prefer to follow the &quot;smoking point&quot; advice, you switch to a different type of cookware.