Hmm, normally if something is mandatory, it will state so somewhere near the field, either in writing or via an asterisk with a key at the bottom of the page.
I've had a look on the Colissimo / la Poste website, but I can't read (understand) French and not everything gets translated into English via Googe's translate tool, so I can't say for sure if it's mandatory or not.
I did go through the entire online customs declaration form via La Poste with none of that info being mandatory, then realised Colissimo is a different branch, and didn't spot a similar online tool from a quick lookover (the online tool, if available, should make it very clear if the fields are required).
On this page, there is a link to a PDF called "Specifications for the CN23 customs declaration form" which is in English and I scanned through that, but couldn't confirm if what the seller is saying is true, but maybe this will be a good starting point to check into it.
https://www.colissimo.entreprise.laposte.fr/en/tools-and-services#emballage
Site is best accessed in Chrome for easy, full-page translation
Alternatively, you could try messaging the relevant postal service on Facebook to confirm - I've had cause to message Chronopost a few times (kind of like the French version of Startrack), and received prompt assistance each time. TBH it sounds like a very strange requirement, considering not all countries have "national IDs", and in quite a few that do, it's not compulsory.
