Andrea, technically the difference in French is as noted by Chris in the previous response. 

I will restrict to just foodstuffs here in general before repeating comments on the use in the lesson.

Foods/drinks that are predominantly or entirely made from 1 main ‘ingredient/substance’ will usually be ‘ … de main ingredient’, eg ‘jus d’orange’.  There are exceptions - ‘ sauce tomate ‘ is just that, although ‘coulis de tomate’ or ‘jus de tomate’ are usual.

On the other hand, something made with several ingredients/substances where 1 (or more) are flavourings or accompaniments will generally be ‘ name of dish’ … à la/au/aux (flavouring/accompaniment) eg poulet à la crème et (aux) champignons

 Compound nouns formed with prepositions à/de/en in French

The problem in the use in the exercise referenced previously is that a true ‘glace’ is not made entirely, or predominantly,  of chestnuts so ‘glace de marrons’ is an unusual description. (Maybe it is meant to be a modern vegan/vegetarian ‘glace’ - if so, that context would be useful, and I would expect that to be described as ‘glace de crème de marrons’).

As I noted under the previous question, my wife (French) has never heard the term abbreviated to ‘glace de marrons’ to refer to "marron" ice cream, and immediately said ‘ glace aux marrons ‘’ when asked for a translation. 

Additionally, searching many French recipe sites returned no recipe ‘glace de marron(s)’ but many of the type ‘ glace à la crème de marrons’ or ‘glace aux marrons’. 

https://www.marmiton.org/recettes/recette_glace-aux-marrons-sans-sorbetiere_19469.aspx 

https://www.cuisineaz.com/recettes/glace-a-la-creme-de-marrons-facile-60425.aspx

Finally, none of these should be confused with the very common ‘Marrons glacés’ .

 https://www.curiouscuisiniere.com/marrons-glaces-candied-chestnuts/

I would almost translate glace de marrons as "iced chestnuts", noting, however, that "iced chestnuts" would rather be "marrons glacés". So glace de marrons is a weird one. Maqybe it is a new creation which uses a chestnut base for the ice cream rather than a milk base? One has to be prepared for everything in this vegan day and age. ;)

It seems to me that there are exception to the rules: for example when describing Gaviscon ( medication for stomach) I find it said and written in two ways. the first is Gaviscon en bouteille and the other Gaviscon suspension flacon and/or en flacon. Any ideas/explanations please, with thanks.