Yay. I wasn’t imagining things. Of course that does rather beg the question of what defines a door being concealed rather than secret.
A question that sages have pondered lo these many long years . . .
Again from p 97:
Concealed doors . . . are doors which are hidden in some way - behind a curtain, covered with plaster, a trap door under a rug, etc. They differ from secret doors in that once their concealment is uncovered they are obviously doors.
Secret doors . . . are portals which a re made to appear to be a normal part of the surface they are in.
So the difference seems to be whether the portal itself is of a hidden nature, like the classic bookshelf passageway in an whodunnit (a secret door), or its a normal door that has been screened.
Personally I think the "covered with plaster" example is not a very happy one, as it makes me want to know how thick the plaster is, at what point plaster becomes the same as the bookshelves or stone flags that typify many secret doors, etc. This issue came up in the original ToH run-through at Origins - what follows is from Alarums & Excursions #4 September 1975:
Finally we entered the central entrance, and headed down the 20' wide passage. The walls were plastered and covered with murals. 40' down there was a picture of two dogheaded beings holding a coffer- which stuck out of the wall. I decided that we could do without Anubis's kin and continued. Our elves reported no secret doors or traps. Ten more feet and out #2 and #3 fighters fell into a trap and lost minor bickerings with three poisoned, 5' spikes each. Poor quality elves we have hear, thought I. We dragged them out. Should our Patriarch raise them from the dead? After 5 hours in the Barren Lands my condition might have been described as numb. With a hazy idea of saving the spell for later I ordered them dropped back into thee pit for later recovery. Neither the Paladin nor the Patriarch protested. The Dungeonmaster did not tell them they should have (both were neos.) No one suggested that we take their useful equipment along with us (one had a bag of holding.) At this point I ordered a Locate Traps spell used- a bit late- and we avoided two more pits on the way down to the end of the corridor- 60'. Still plastered walls, still the elves detected no hollow spaces or hidden doors. What a time to pick defective elves! . . .
We got out along the crawlway. The room was plastered, the elves detected nothing and I had not yet grasped that a 1/2" of plaster was elf proof. Gygax's elves have to see secret doors, reasonable but not what I am accustomed to. . . .
Someone else had a brilliant thought- what's behind the plaster? We broke some and found a door. Blasted insensitive elves!
How this relates to, or is consistent with, the DMG text I've quoted is a problem I'll leave for someone else to resolve!