By "haven't been online" he means cubers who probably haven't been on this website in years (which can easily be seen in the ZZ/ZB discussion thread from.....2010?!?!?!?!?) and they won't reply.
alright so let me try to explain:
Bumping is basically replying or making a new post to an old thread. This is only bad if you add irrelevant content.
Example: replying “cool” to a thread that has had no new replies since 2020 is a bad bump.
tip: there will be a date where the last post was made, so you can determine whether it is a good idea to bump it or not.