If Leif forgot to tell anybody about it, then how come we know his name in this connection? The records of his doings were left, and even disseminated, but not too widely. His literary agent was none too good, that's all.
Same with Gregoire Mendel ~ he didn't publish!
Another guy came up with what amounts to the Quark Theory back in the 1700s. However, it didn't explain chemistry, and it took another 200 years for someone else to come up with the same theory. By that time we knew what it meant (if not all the details).
We don't count Leif any more than we count those ancient North Africans.