[Will Wright]

As for Will's "A coffin is not a locked room" I thought it was a reference to one of Ellery Queen's novels, that has almost the exact same quotation.

Spoiler:

Ellery:"Explain the locked room? Why should I? A coffin is not a locked room."

That referred to the victim not dying inside the locked room itself, but being moved after his death. It was where the victim was supposed to be after dying. In other words, every victim had a perfect alibi if the victim died when everyone thought he did. But he died at a different time, and the coffin was what made the room seem impossible.

Ellery goes on to deduce who the criminal is based on that, and chases after the criminal.

Then he explains the trick involving a fake key in order to explain how the murderer made it seem like the room was locked even though the key he found was not the one used to lock the door.

It's a complex trick.

So yeah, that's what I took it as. Almost all of Will's responses were, word per word, a quotation from some Golden Age detective. I just automatically assumed the same tricks were used for those locked rooms as the locked rooms/quotations were very similar.