If I understand this correctly, it is a "Limited Pre-Order of 2,100 Units", not necessarily a limited edition of 2100 units...
nice wording marketing/promotion thought...

So it is not limited to 2,100 pairs. They can reproduce it after pre order? 🤔
up to 21M

I am not sure if it is -or not- limited to 2100 in total. This is just speculation according to the phraseology used in their X (Twitter) promo & listing description...
If the shoe fits (no pun intended), we will be able to attach our Ledger devices as a Jibbitz/charm

To answer your question I wrote to them and this is what they said..