The correct explanation for why some people use 1918 as the starting date of the Russian Civil War is:
It was the year that the Russians got out of World War I with the Treaty of Brest-Litovsk, so they could focus their attention elsewhere.
This event marked a significant turning point for the Bolshevik government, allowing them to redirect their efforts towards dealing with internal opposition and consolidating power within Russia, which ultimately led to the onset of the civil war.