While you make a good point (that one always needs to ensure the facts they know/take for granted are correct and haven't changed), it doesn't seem that is what he is referring to.
The need to reread and reconsider books, ideas, etc. is not to check if the facts have changed, but to make sure you didn't miss a point because of lack of knowledge or bias.
For example (on a very basic level), I find that if I reread a book on a programming language after using it for some time I notice things which I missed on a previous reading or wasn't able to appreciate due to lack of familiarity with the language. The same can apply to history - or any other study - where one only appreciates certain details after understanding the larger picture and surrounding events.
For this reason there lies an advantage in both rereading the same book (immediately or after some time) by itself and reading after studying more on the topic from other sources.
The need to reread and reconsider books, ideas, etc. is not to check if the facts have changed, but to make sure you didn't miss a point because of lack of knowledge or bias.
For example (on a very basic level), I find that if I reread a book on a programming language after using it for some time I notice things which I missed on a previous reading or wasn't able to appreciate due to lack of familiarity with the language. The same can apply to history - or any other study - where one only appreciates certain details after understanding the larger picture and surrounding events.
For this reason there lies an advantage in both rereading the same book (immediately or after some time) by itself and reading after studying more on the topic from other sources.