You can keep saying that all you like. The books
explicitly say otherwise. I'm going to believe them over you.
No, it isn't. The page you are referring to is when the DM has to INVENT a new DC for something that didn't already have one. It is not for deciding whether the wooden door in front of the players is a level 30 wooden door or a level 2 wooden door. Because,
even in the 4e DMG, doors had specific, defined DCs (4e DMG page 64):
Strength Check to... | DC |
---|
Break down wooden door | 13 |
Break down reinforced door | 16 |
Break down barred door | 20 |
Break down iron door | 23 |
Break down adamantine door | 27 |
Break through force portal | 30 |
Force open wooden portcullis | 21 |
Force open iron portcullis | 28 |
Force open adamantine portcullis | 35 |
So you are just, straight-up, wrong.