The boat could have easily had a marker in it..and he was a student so could have carried one with him....I see no problem that he could have left a confession....they all want to go out in a blaze of glory!...Leaving their “mark” before they leave. It is standard suicide bombers make videos of themselves etc. So yes it’s likely he left this note.
It said he found a PEN in the boat.
Sorry, just not buying this story. That’s just me!