I don't know the details, but a movie that is shot "flat" is filmed open matte. The actual film frame is about 1.37:1, and is cropped down to 1.85:1 for theatrical exhibition. Warner is very notorious for presenting such films on DVD fullscreen, instead of in a properly-framed widescreen transfer.
I'm not sure what "flat comedies" means, but can help
out with "flat" pertaining to "Dial M for Murder".
In the industry, "flat" meant one of two things.
1) If a film was shot with an anamorphic lens it is
typically known as a 'Scope' (widescreen) film. A standard
film (1.37:1 or some softmatted films) were often called
a 'flat' release.
2) 3-D films shown with only one view (using only the left
or right view) is now in 2-D, and hence is called called
a 'flat release'. This term has been widely used by the
industry from the 1950's through the 1980's.
Sadly only term #2 applies to Dial M, which seems to mean
only a 2-D version will make it to DVD. That's a real shame,
considering both a 3-D and a 2-D version could fit on one disc.
Afterthought and slight clarification:
"Flat" *could* apply for Dial M, as meaning an open matte
film, but to go out of the way to point out that it was an
open matte shot film when so many other films mentioned
were also 'flat' makes no sense using that meaning. But if
only a 2-D version would be offered on the DVD, then the
second term makes more sense.