It's ridiculous in the novel too. (Note that it wasn't me who came up with the "Christie had run out of ideas" defense.)

Sayers' crimes often don't make any sense either. What makes her a better writer is that her novels are also about something else, so the reader can ignore the parts that don't work and still get a good story.

The difference between "not as good as Beethoven" and "not as good as Sayers" (or at least "not as good as Sayers at crime plotting") is that "not as good as Beethoven" can still be pretty good.