Here's the copy on the front of B2, emphasis mine:Just to be stupidly pedantic - Keep on the Borderlands absolutely was intended as a starter kit to customise.
I wonder if that might explain some of the difference in approach here. Early, 1e and 2e modules especially, absolutely expected DM's to modify and flesh them out. To the point of deliberately leaving areas blank or giving suggestions for what the DM might do to flesh it out.
I'd almost argue that it's the Adventure Path modules that have been presented as complete and containing everything needed to run them. Which is something kinda new in modules.
"This module includes a cover folder with maps and a complete description booklet to form a ready-made scenario for DUNGEONS & DRAGONS® Basic Set. It has been specially designed for use by beginning Dungeon Masters so that they may begin play with a minimum of preparations."
There have been a handful of "fill-in-the-blank" modules (B1, Ruins of Undermountain), but IMO the vast majority have been intended to be ready-to-play as written, and this is by no means something new.
Of course all modules require some degree of "fleshing out." Every possible action the player's might take can't be anticipated in advance. That's very different from completely rewriting them or pulling bits and pieces from them as inspiration.
The notion that Dragon Heist is not meant to be played as written- I just don't get it? I mean it is notorious for its railroading. Is this some kind of trick by the author? When they say "don't let the PCs get the MacGuffin whatever you do" they really mean "do whatever you want! It's a freeform sandbox for you to make your own!"