Not only --from=si is broken. Also --to=si is broken:
$ numfmt --to=si 3000
3,0K
In order to not break backwards compatibility, you probably have to
introduce a switch --lowercase-kilo such that --to=si produces proper SI
compliant output. Then have --from=si accept both uppercase and lowercas
So Pádraig's patch does allow for parsing lowercase k, but it does not
change numfmt to use lowercase k in its output in si mode.
As Pádraig has shown, ls uses lowercase k in --si mode. So it uses
lowercase k for 1000. I think that numfmt should behave the same for
consistency reasons.
Also,
