Neat article, and good advice.
One potential problem is when you have your example DM say, "We can always go over the details later." I agree that front-loading the conversation often turns into the DM just delivering exposition, but if you and the players have different ideas of what those details are, it can be hard to backtrack.
For example, the players may decide to go and assault the demon in the Nine Hells, rather than summoning it to the mortal realm and risk it getting loose. However, if the location of the demon's lair in unknown to the sage (or the sage managed to keep that information hidden, despite the intimidate roll), your players may get halfway through preparing to storm the demon's realm, ask you for some clarification, and put you in an awkward position: Do you,
a) Tell them anyways, opening the door to fridge logic ("Hey, if the demon's secret lair is so easily found, why has the order of demon-killing paladins not gone there and destroyed it?")?
b) Tell them they didn't get that information, leaving them possibly feeling cheated (especially if the sage did the sensible thing and skipped town, making him unavailable to answer more questions)?
The only solution that I've come up with to this problem is just an understanding between the DM and players, reached through time and established as a rule of the table. For example, I'll tell the players the kind of information and level of detail, making sure that we're all on the same page.
Example:
"The sage tells you a bit about Barunor -apparently it's vain and a master of deceit- and that the demon can be summoned by speaking it's name while staring into a mirror and standing in calf's blood. He's shaking in his fancy boots, but doesn't offer anything else. Anything you'd like to ask about?"
The players know that this is their chance to ask for anything they might want; if not, there's no retconning. If they ask about something that would grind the session to a halt (for instance, details on the demon's lair), I'll tell them the general level of detail they can get ("the sage doesn't know much, but he can tell you where a portal to the Nine Hells is, and that the demon lives in a sprawling castle with vast evil gardens") with the understanding that we'll go over it as soon as the current scene is finished.