Ah, you want it to tile them but at 100% size. It does look like even though the canvas size is small enough, that the print driver is getting the notion that a bigger document is being printed and scaling everything accordingly.
Well, this is OmniGraffle, there's always a way around any problem, right? :-)
tell application "OmniGraffle Professional 5"
-- set up export options
set current export settings's area type to current canvas
set current export settings's copy linked images to true
set current export settings's draws background to true
set current export settings's export scale to 1.0
set current export settings's include border to true
set current export settings's resolution to (300.0 / 72.0)
-- get a destination folder
set dstFolder to POSIX path of (choose folder)
repeat with _canvas in every canvas of front document
set canvas of front window to _canvas
save document of front window in POSIX file (dstFolder & name of _canvas & ".pdf")
Running that script will prompt you for a folder, and export all of your canvases there as PDF files (<canvas name>.pdf). Open up a new document in OG, drag in the PDF files and lay them out on the pages how you want (you're doing the step the printer driver and OG couldn't manage to do the way you wanted), and print.