Just digging up this thread, because the patent for this lens materialized. The linked article of from a Japanese site asobinet.
https://asobinet.com/info-patent-ricoh-21mmf2-4/
It's quite normal for different variations to be patented along with the one that becomes an actual product. These are almost certainly included for completeness and not with the intention of actually making them.
In this case, they also included:
- 18mm f2.4 for DSLR
- 14mm f2.4 for mirrorless
As I mentioned this does not mean either lens is coming or that Ricoh is working on a mirrorless camera. It just means that they are securing the IP from a legal standpoint.