From a quick glance Don Ho looks to be based in France, and a search of their trademark registry gives a current result for Notepad++: https://data.inpi.fr/marques/FR5133202
On closer look, I think you’re right. I also tried searching the USPTO trademark database by author and by product name and couldn’t find anything. I don’t know why I thought he had trademarked it. Maybe it could qualify as a common law trademark, but I’m really not sure as this is well outside my area of expertise.