tbh, databases of any kind could be used to make a chat application if your query structures are well setup etc.,.
But the feature you are mentioning like having threads could be easily be solved using like a document store ex; mongodb or any other no-sql db that could help you save time in both setting up things and querying.
really curious about if all the people that do leet code can transfer those skills to their job. Problem solving is a good metric i suppose, but how good?