You are correct about both the limitation in Resize and the lack of said limitation in Imposition.
You are also correct that this is funny...
Luckily there's a pretty simple way to get around this. Use preflight to check the document dimensions and then route based on the preflight result.
The Page Size preflight check (in the Custom Checks) should be able to perform checks that are analogous to those in the job characteristics
One thing to keep in mind: If the check is true then the status will be set to either warning or error (depending on what you specify in the Acrobat or pdfToolbox preflight profile)