I think Wikipedia is the only place where an early employee who left relatively quickly would be considered a "co-founder". If this was a tech company i dont think Larry would be considered a co-founder.
I think the thing is a soar subject because Wikipedia essentially rejected all of Sangar's ideas, but he's still kind of riding on its coattails.
A founder only has to be there for the beginning. "Early employee" and "left relatively quickly" would not disqualify him.
I know little about Sanger but he wouldn't be the first person to have been written out. Elon Musk's partner in early PayPal suffered that fate.