Is Samael the true final boss if anda continue to play after beating Abaddon?

I played on Apocalyptic mode, beat Abaddon, credits rolled, then it went to the main menu. I loaded the saved file right before I beat it, which had the health I had when I beat him, joined the fight again to see if there was anything different, and it just loaded the video of him crawling away, being a loop. So, as far as I know, no.