One can use binary prefixes (iB, e.g. MiB) with dd command, but that is
not written in the help text.

I just found out because I wanted to add support for binary prefixes
since I did not know they are already supported.

It's a hidden, but valuable feature, since I think, that M is binary is
hard to remember and can easily lead to confusion.

Much clearer is to use MiB. "M" is mentioned only for backwards
compatibility in this patch.

The parsing for binary prefixes is done in

gnulib/lib/xstrtol.c

so this is not dd specific.