They used that phrasing because that's how Amazon's PR people described the situation to them. The reality seems to have been more like two cofounders than boss + employee. Though I would not be surprised if Shel was actually a cofounder in the strict legal sense as well, meaning that he got stock at the time the company was founded. I'll ask him next time I talk to him.
ojbyrne would take issue with you calling Kevin Rose a single founder. ;-)
A bunch of the other ones seem suspect too. Wikipedia lists the founder and first CEO of Lycos as Bob Davis - Michael Mauldin invented it as a research project, but if you go by research project inception and not incorporation, Larry Page is the sole founder of Google.
Shawn also had an incredible amount of help from Jordan Ritter and Ali Aydar before the company was even a company. He also would have been a lot better as a single-founder, as his co-founder uncle John Fanning took advantage of his inexperience.
...which was around when Berkshire Hathaway turned from a textile manufacturer to an investment vehicle.
It's hard to lump financials in with tech startups, though. Technically, when he Buffett started the Buffett partnership, he had lots of cofounders. The structure of an investment fund is a single managing partner (who makes the decisions) and then lots of limited partners (who put up the money). In the eyes of the law, they're all cofounders.
If you go by practical instead of legal definitions - who provided the know-how? - then Buffett's early partner was effectively Benjamin Graham. After all, it was at Benjamin Graham's firm that Buffett first learned about investing, and Graham served as a mentor long afterwards.