Twitter could, of course, have just given a warning. But I also don't see any problem treating the tweet as if it were current. The age of the tweet does not change whether it is acceptable or not, does it?
Getting a message saying something like "this message violates the rules today, so we're going to delete it" or "delete this message within a week or we'll have to ban your account" would seem more reasonable to me than immediate ex post facto enforcement.