Searching for “Calle 18 revolucionarios” yields: https://cispes.org/article/special-report-el-salvador-enacts-emergency-security-measures-against-gang-violence?language=es

So apparently there are in fact two groups: “Calle 18-Sureños” and “Calle 18-Revolucionarios”

To me it seems that “18th Street Revolucionarios” is just the name that this one gang has given to itself. Note that when the article calls the two individuals “Revolucionarios”, the word is capitalized, which confirms that it’s a proper name rather than a description.