It's historical reference for how the rule evolved. Before "bumps" were a thing that needed a rule, we would not explicitly remove them at all time. That's because there are no strict rules for things that are not a problem and many years back, it was not a widespread occurrence. Then we started specifically targeting spammers who bumped more often than 24 hours as a "guidance" to moderators. Still, the rules never said anything about bumps but moderators instructed those users directly when they received a warning, specifying that they crossed that mark.
Then over time bumping became so common that some threads consisted of 8 pages of posts but after removing "bumps", only 2 pages remained. "bumping" was also less needed - we have less posts per day than at the peak popularity of Open Tibia a few years ago and therefore threads don't fade away that soon anyway. We also increase the standards - we don't need useless "bump" posts at all.
All of this is a result of action taken by Staff in response to problems as they occur. We don't need an explicit rule stating what particular letters make a "bump" and how often exactly you can do it. Almost always it will be deleted now and depending on how frequently it happened, you may get a warning. This is all under the "No spamming" rule. If your post is not valuable to the reader, then don't post.
In fact, there are MANY types of "covert bumps" that we discover as well that are considered "bumps" AND "spam". Imagine this:
"Server is starting in 5 hours!"
"Server is starting in 4 hours!"
"Server is starting in 3 hours!"
"Server is starting in 2 hours!"
"Server is starting in 1 hours!"
"Server is starting in 30 minutes!"
"Start has been rescheduled for 19:00 due to low number of players!"
"Server is starting in 3 hours!"
...
Would you say those are bumps? It doesn't matter, they are certainly spammy posts that are not worth anything to the reader. We get the concept of time and we know how much time there is to server start because the date/time are shown in the thread. Yet people are surprised. We know exactly what they're doing - trying to make the thread look "active" and the server "interesting" by generating posts. And they are not same content you may say! Sure, the number is changed, maybe the sentence is changed too. Again, think about the value to the reader. If someone finds those posts valuable, then they probably don't know how to use a simple clock and probably unlikely to be able to play this game.