Back in the 80s and 90s rhey would advertise products on TV with "6 to 8 weeks for delivery".

Now I wonder if they did this to batch up a manufacturing run once enough orders were received.

Totally. You also get batch efficiencies shipping 10k orders in a day vs dribbling them out over weeks, and you can use sub-standard shipping methods that are cheaper because… the carriers themselves are also batching the work.