You are correct, that phrase as stated would mean no matter how little impact you think 45RPM had, you estimate would not be too low. The author surely meant '

You are correct, that phrase as stated would mean no matter how little impact you think 45RPM had, you estimate would not be too low. The author surely meant 'impossible to overestimate.' --- I don't know if this is a common mix up, like, for example: "I could care less." (which means you have not reached a minimum of caring) vs "I couldn't care less." (which means you have reached minimum of caring) --- Or the more pernicious (in my opinion) and pervasive: "Not all X are Y" (some X might be Y, but at least some are not) vs "All X are not Y" (There are no X that are also Y)