Should this be taken literally, or is this under-promise, over-deliver?
Buffett has been saying this almost verbatim at least since the mid 80s. I wouldn't take it literally but as a prudent reminder to keep expectations modest.
I much prefer it to management teams offering confident forecasts of shareholder return of 15% over 5 years etc.
Berkshire will deliver a 10% or so return in almost all market environments over a 10 year period ( more if you enter during a weak period). Good enough for me as a bedrock holding in a portfolio.
Amidst all the criticism of Berkshire performance etc, I just did an IRR calculation on my position for the last 4 years. With significant buys in Aug-Dec 2015 and July 18-Feb 19, the IRR is 17.4% in GBP and 11.7% in USD. And this with an endpoint where Berkshire has been flatlining since the new year, is sitting on gobs of cash and the business is chugging along well.
I'd take it anytime.