The parent comment was(pretty clearly) referring to what you're calling "flow-level logs"... Quite obviously you can delete the router's logs(it's mentioned in

The parent comment was(pretty clearly) referring to what you're calling "flow-level logs"... Quite obviously you can delete the router's logs(it's mentioned in the article and by the parent comment); the ISP's traffic logs on the other hand? > those are so voluminous that they probably aren't kept long. Bold assertion.