That script doesn’t actually import anything into EagleFiler. It’s to prepare the messages for importing if Mail hasn’t fully dowloaded them.
To import only older messages from Mail to EagleFiler, you could create a smart mailbox in Mail to show the desired date range and then select the messages and press EagleFiler’s capture key.
Maybe this could be part of a Feature Request to create a way (maybe AppleScript) to auto-archive old email messages?!? (not files or other capture items)