Yeah, there's not anything else that we have extant that shows anything like the extreme treatment the spartans gave the messenians. Granted it was mostly recorded by biased folks anyway, but still you can tell they thought it was extreme (though they probably hadn't seen what hell was really like for those poor chaps in Laurion either). It's the only time I know of that greek slavery got close to that american south slavery too - in the way that it's a whole group (a division of an ethnicity) that was treated this way, not just folks who had a bad day and had their city sacked or got nabbed on the beachside somewhere.