As much of a joke as it may be, it's still against our rules. We want to maintain a sense of consistency in the enforcement of our rules, and even if this case was almost certainly a joke, there are some cases where it's not so clear, and if we allow some unclear jokes drawing the line between joke and seriousness becomes much harder. If it was some very clear joke like "give me stone rank" it would be one thing, but when you're saying the exact same thing as someone who actually wants a rank, and the only thing we can do to know intent is to infer, it's really hard, especially if it's not a well-known player whose sense of humour we are not familiar with .
We also would like to avoid overturning punishments "because it was a joke", as we are afraid it will turn into a (even more common that it is now) excuse to avoid punishment.
Now, this obviously does not mean we are against jokes, and it does not mean we disallow sarcasm and irony based humour, but this specific ocurrence was written in the exact way a normal player asking for a rank would, without any hyperbole or any kind of indirect indication that would let us know that it's a joke, and so we concluded it's reasonable to keep the warning, even if for the sake of consistency.
Finally, it's possible you could be asking for a rank to your friend, an alt, or even plus rank for your current account, so it's still possible you could be actually asking for a rank, even if it's not our main reason for keeping the punishment.