https://arnoldzwicky.org/2009/06/03/the-decimators/
The meaning may be obscure and obsolete, but the word does mean something specific.
Rather, I'm sure Caesar used the appropriate conjugation of the Latin verb "decimō".
Two separate, but related, words. With two separate, but related, meanings.
https://arnoldzwicky.org/2009/06/03/the-decimators/