Not a teacher, 53-year-old American.
There is an American song which contains the line, "You ain't seen nothing, yet." The point of this line is that what has already happened is merely the beginning (the tip of the iceberg) - there is much more (bad/good) to come.
I think he was specifically trying to work this line into his statement - and chose the tense of his statement to match the "seen nothing yet" portion.
That's my opinion.