Weak Strike's On Buffalo Nickels

In the Coin World's grading book, it states the following regarding Buffalo nickels:

"The weakly struck coins lack designs details, even on uncirculated examples. Because of the prevalence of weak strikes, it is helpful to keep the following years and mint marks in mind: 1913-S, 1919-D & S, 1920-D & S, 1921-S, 1923-S, 1924-D & S, 1925-D & S, 1926-D & S, 1927-D & S, 1928-D & S, 1929-D, 1931-S, 1934-D, 1935-D, and the 1937-D three legged bison."

They really just say to keep these dates in mind because there is a prevalence of weak strikes in Buffalo nickels.

So, do they mean that these particular dates and mint marks are weak strikes, or do they mean that these particular dates and mint marks are NOT weak strikes and we should keep an eye out for them because they stand out as strong strikes amongst the "prevalence of weak strikes"?