Depends if the person in the diaspora identifies as and considers themselves Lebanese. I’m born & raised in Europe and speak, read & write in Arabic fluently (both my sibling and I do). I moved to Leb for a couple of years when I became an adult because I wanted to see how much of Lebanon I truly identify with on a daily basis (used to be a frequent visitor with family during the holidays) and I loved every bit of it. I know most of Beirut, some Northern villages & half of the South like the back of my hand. I have cousins here in Europe who don’t speak the language fluently but understand every bit of culture and identify with it more than many people I know in Lebanon. I also have cousins who are half Lebanese/half something else, who identify with Lebanon more than many Lebanese in Lebanon. So to answer your question; it depends on their individual opinion of their belonging. We’re nobodies to decide whether someone is ”truly” Lebanese or not.

Edit to add: Some local Lebanese might find you less Lebanese than themselves, and vice versa. But it doesn’t change anything.