According to Canva, exporting as ZIP is not possible unfortunately.
So, it might not be possible at all via the API.
For more information, see Exporting designs - Canva Apps SDK Documentation, and Exports: Exports - Canva Connect APIs Documentation
Hope this helps!
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!