Hi there, anything labeled Free on the Unity Asset store should in fact be free to my knowledge.
Free and Extension License in combination just means one Free license is required per seat. FAQ is here : https://assetstore.unity.com/browse/eula-faq
Basically anyone wanting to use that Asset needs to acquire their own license by "Add to my Assets", at least such is my understanding, and as such is also eligible for use in this game jam :)