I disagree with your conclusion here, for two reasons. First, because you can rationalize the idea of abetting a romance between a demon and a devil against the backdrop of the Blood War as a simple case of Romeo and Juliet - forbidden love against the backdrop of feuding groups. That's not particularly exclusionary in any edition.
No, it's not exclusionary but if we are being that broad then there ARE no new stories, ever. I think they mean "new stories" in terms of the very specific details of those stories. It's likely this story is actually one that can't be run in 4e at all. Demons want to destroy the universe in 4e. It's unlikely that they'd spend their time cultivating relationships of any kind.
The second reason is that, while a "simple quest to deliver a letter" would be very different from the above, there's nothing about that that 4E inherently does better than any previous edition. The only way to hold that there'd be any noticeable difference would be if you kept all of the factors exactly the same, and noticed what background assumptions were different...and even then, I don't think it'd be notably altered in any practical context. Going to Hell, picking up an item, and delivering it to the Abyss is going to be dangerous for the PCs regardless of whether or not there's a war between those two planes going on.
But the kind of things you'd see in the Abyss and Hell would be completely different in the 2 editions given they are very different places. Also, you might be willing to take the risk given that there is no particular dislike between demons and devils in 4e but you might not want to involve yourself in the Blood War. They'd be different stories in the 2 editions: The reasons to go on the quests would be different, the places you'd visit would be different and it's likely your interactions with the creatures would be different.
That example is just too similar in the two editions. Let's say a story about attempting to find a Elemental Gem created by the Primordials during their war against the Gods. It is now in the elemental chaos in an ice fortress with lava running through it guarded by elementals of all 4 varieties. The fortress can be reached by walking. It isn't environmentally dangerous to the PCs.
That story would be difficult, if not impossible to run in previous editions. The old elemental planes don't really allow that as an option. The reason the planes are different is because the alignments are different so the planes no longer need to be tied to them.
Again, I say that the use of "cannot" in regards to what stories are told is overrated. If you've relegated certain campaign elements to the distant background, they cease to be a factor; this relegation is much easier than people tend to assume. You can have stories about X-Wing pilots without anyone being Force-sensitive. You can set an adventure on a single planet where no one pilots a starship, etc.
Yes, but you can't run the story where one member Galactic Senate on Coruscant can't seem to get off the planet without coming up with a LOT of convoluted reasons why there are no starships around. Yes, MANY stories can be told. But some cannot. That's fine. Restrictions often cause some of the best creativity. However, if you want to open it up to entirely new stories something changing the restrictions helps.
This is true for more recent editions. I'm not sure how true that is for the older ones (my books are at home). I can tell you that OSRIC's version of
detect evil doesn't grade it by Hit Dice or the type of creature detected.
Yeah, I believe it's true for 1e, 2e, 3e, and 3.5e. Though I could be wrong about 1e and 2e. It's been a while. I do remember 2e having some way to detect powerful as opposed to weak evil creatures.