It should not be seperate from the SCP bundle. Likely itch.io acting up again. I had some trouble with the platform last year. Can I have you DM me so I can provide it free of charge?
Viewing post in 2020 Artpack updated with WIP stuff comments
It is different from the 2019 zip as that includes all the art done from 2017-2019.
It's included with all the 2020 bundles. It currently contains all full res images of the art as well as the WIPs and is still getting updated like the game.
It will also get an update in the first quarter of 2021 with the next big update.