There is a new plugin built on the same code base

Download the Squarespace version of the plugin from https://www.moeloubani.com/downloads/import-external-images-sqspc.zip

…and replace:

curl_grab_image($file, $tmp);

with:

curl_grab_image($file . “?format=original”, $tmp);

in the modified squarespace version to get the original attachments downloaded/imported in full size from Squarespace and saved in your WordPress Media Library.

This took me hours to figure out and I found the above solution in https://www.ads-software.com/support/topic/has-anyone-gotten-this-to-work-with-squarespace/ way too late.

I hate Squarespace even more now.