My reading of it is that it requires that there be a real RP reason, not just a hasty justification tossed out there within a week or so of the secession. Again, in 99% of cases, this should be relatively obvious to neutral observers.
I'm not so sure that how long the idea has been around has much, if any, value at all. Just because you've been planning it for a while does not make it legitimate. Nor does the possibility that you've only thought it up 5 minutes ago mean that you're breaking the rule.
For example, you could have been planning for years to spin off a new realm that would make absorbing all of Enweil much easier. Does the fact that you've been planning it for years make it OK?
Or the duke could have been insulted by the ruler, and decided on the spur of the moment to secede. Does the fact that it wasn't planned long in advance make it a violation?
Nor is the presence or absence of IG RP a determining factor. You can RP the hell out of a situation, and still violate a rule. That's because this rule is an OOC rule, which regulates the behavior of players. A rule such as this may force a player to alter the behavior of their characters in order to comply with the rule. Sometimes it may make perfect IC sense to do something *right now*, but OOC rules/considerations may require us to alter those actions to comply with the rules. That's just something that we, as game players, have to deal with.
IMHO, I don't think you can judge this rule solely by intent. It is quite possible to unintentionally violate a rule. The results of what you have done should count just as heavily as the intent with which you performed the action. Simply allowing someone to say "Ooops, I didn't *intend* to do that" is kind of ridiculous.
As for the idea that this particular secession does not create any specific advantage, it certainly does. I can think of two or three without even trying very hard, and none of them require the realm to be particularly strong, or have very many nobles.
And this entire situation could have easily been avoided by simply delaying the secession until the war was over.
All in all, this particular rule is nearly worthless, both in the way it is being interpreted, and in the way it has been enforced in the past. I can think of at least two secession cases on EC that were ridiculously blatant circumvention of multiple game mechanics, including the recruiting mechanic, that were shrugged off as OK because there was some RP behind it.