Yes, decimated comes from how Romans would randomly execute 1 out of 10 soldiers after a rebellion. They still need an army so they can't kill them all.
But words evolve and decimate has also come to mean entirely destroyed simply because that's what people erroneously thought that it meant.
The wordsmiths do not dictate how our language evolves, they merely report on how we decide to use it.
For example, they had hissy fits when people said; "I'll fax it to you."
"Fax is NOT a verb!" They sputtered.
But Fax IS now a verb. Because it is we who decide, not them.