This has a lot more credibility than the outlandish claims of some in here. The article says he was challenged by Terrell Davis, so I assume it was tested by somebody that knew what they were doing and was well documented. It also shows that the 5.94 was his best time at age 47. He had been doing this for ten years with times as bad as 6.77 at age 37.
Doing the math, 40 yards/120 feet. A 4.5 time is about 27 feet per second. That means 2.7 feet per tenth of a second. So you guys claiming you could do 4.5 or so 40s, you're saying you could stay within 3 or 4 feet for 40 yards of the fastest NFL players?