"Intuitively obvious" like his old math teacher did, namely that it is obvious, if you know which clues are important and have to be combined. Thus, it's certainly meant in a funny, faked presumptuous way to imitate the way a teacher would speak to his pupils, who try to figure some problem out. But it's not a randmon joke meaning that the killer isn't obvious at all, or that there isn't a line of reasoning, which makes the suspect more obvious that all the other suspects.

As said, the reasoning for Graendal isn't really more obvious than the reasoning for Sammael or other Forsaken before LoC. Therefore - and for all the other reasons I often pointed out - it's very unlikely that Graendal is the solution.

On the other hand, especially for the author (the "math teacher" in the analogy), who of course exactly knows what Slayer is, can do and did before, and who is aware that there are important clues to figure this out in the first 5 books (plus additional indirect clues in the next books), it's obvious that the Shadow's assassin killed yet another betrayer.

Personally, if it turns out to be Slayer, I think one can't disagree with RJ that it was in a way obvious, following my line of reasoning. But we will RAFO!