I have never personally heard Saotome Sensei say 20 years with the founder. I've heard him say 15 years he knew O Sensei. I think the confusion some people may have made is this: 1975 (the year he left Japan) - 1955 (the year he met O Sensei) = 20 years.
When Saotome Sensei stated "I practiced close to O Sensei - you can't imagine- for 20 years, I have these memories..."
(same source) was he accurate?