Hello and welcome @Stephane_Gonthier 
That notification is for moderators for when there have been items in the Review Queue for a certain amount of time. There are a couple of settings for the different versions so you can choose what the time threshold is for each - notify about reviewable item after (flags) and notify about queued posts after (things that need approval).
But neither would notify you about topics that have not had a reply yet. I think you would need to run a data explorer query to get those.