It could hold long enough to do the job. The buckle could be there, but just as a loose end; using the strap alone as a noose.“A nylon strap with a plastic buckle dangled from a tree branch about 10 feet above him, leading deputy sheriffs to conclude he hanged himself.”
How long was this strap? If it’s a plastic buckle it won’t hold the weight of a human for long. Even if it did, the Florida sun & heat would deteriorate the plastic quickly.
Did Ian tell his family he was traveling to the US or Florida in particular? Seems like a long way to go to commit suicide.