To concede that duration can have an influence is one thing, but to assert that it definitively determines the seasonality of a trip is an oversimplification.
Here, in the United Kingdom, for example, seasonality is often dictated by school holidays, which forces many families to travel during peak season, even for short stays, regardless of their budget.
The flexibility of dates is a much more determining factor for enjoying the low season than just duration.
A solo traveler can find cheap flights and hotels for two weeks in November, while a family with children will pay a high price for a week in August.