Changing either/or properties of built-in kinds

The Inform manual says:

Yet it appears “we” can do that. I try it – and it works. The index updates appropriately to indicate this changed state. Trying it out, changing the either/or property does seem to work, as can be seen in this simple example:

[code]A door is usually open.

Stage is a room.
The portal is a door.
The portal is west of the stage and east of backstage.
Backstage is a room.[/code]

Try it with and without the “A door is usually open” line.

So is the manual wrong? Or is there some subtlety it’s not making clear? There’s no indication at all in the manual as to why Inform would not allow it and it would seem odd that this wouldn’t work especially since earlier in the same chapter it says “But if we want to rearrange Inform’s default ideas, we can do that quite easily.”

It’s surely a miss in the update of the manual. Later assertions of the default value of an either/or property of a kind has been allowed to override earlier ones for five years (since build 4U65), it appears.