There are a few inconsistencies in the way the Import & Export links will change from simple to AT sites depending on the state of the Dashboard Appearance toggle.

How it works on simple sites:

By default, both the Import & Export links will take to Calypso.

If we activate the Dashboard Appearance toggle, the Import link will take to /wp-admin/import.php but the Export link will still show https://wordpress.com/export/

How it works on Atomic:

By default, the Import link takes to Calypso, but the Export link takes to wp-admin/export.php.

If we activate the Dashboard Appearance toggle, both menu items will take to the wp-admin equivalent.

How to improve this on simple sites?

I would advocate that if Dashboard Appearance is active, "Export" should take to the WP-Admin equivalent since this is what users would expect and by not doing this we remove any direct access to this exporter.

The season for this is that currently there is not 100% parity between them. for instance, the Calypso exporter will not let you export only Portfolio or Testimonial CTPs:

How to improve this on AT sites?

I believe that given the way the Dashboard Appearance toggle is supposed to work, having it off should take you to Calypso, and having it on should always show wp-admin.

Unfortunately, wordpress.com/export will not work on AT sites. If I visit wordpress.com/export/atomicsite.com I will get this message:

Given this, we will just need to accept this inconsistency for AT sites.