They are interchangeable. I can’t recall any example that suggested they weren’t, although the questions may only provide the option of one of the correct answers rather than both for the 3 relevant seasons. 

If you have found one to do otherwise, worth reporting directly though. 

Edit: to clarify, and after some further reading, the interchangeable use discussed here, applies when saying “IN summer/winter/autumn”.

There are other sites that discuss the use of other prepositional constructs and specific modifiers, including “à l’ “, “ dans l’ “, durant, pendant etc

For simplicity, for non-native speakers, some recommend restricting to using ‘en ou au’ to mean “IN (season)”.

https://www.fluentu.com/blog/french/seasons-in-french/

This link is a Canadian site :

http://bdl.oqlf.gouv.qc.ca/bdl/gabarit_bdl.asp?id=3516

Céline - I think we are talking a bit at crossed purposes. 

In the context discussed in the lesson, the ‘en’ and “ l’ “ are interchangeable, but that is not always the case when referring to the seasons, at least as I understand it. For instance, I don’t think you can substitute en for l’ in “ début de l’hiver ...”. 

I am not sure if Brian is referring just to questions meaning “in summer etc” or other questions with constructs in which they are not interchangeable.

https://www.druide.com/fr/enquetes/quelle-preposition-porter-cette-saison