That was definitely other than a normal Cero, you can tell by Isshin's confused look and he wondered if it was a cero. It's evident that he's seen a Menos' Cero before because he tried to relate it to that, but I guess at that time, they didn't know anything about Gran Ceros nor Cero Oscuras. I'm not saying that it was that powerful, like the cero he used against Ulqi, but it's not an ordinary cero.