So this claim seems well sourced, but I had to look it up because my bullshit detector always perks up when Sparta is discussed.

The surviving written sources are limited and largely from a non-Spartan viewpoint. Anton Powell wrote that to say the written sources are “‘not without problems’… as an understatement would be hard to beat”.

By Roman times, when a more organized and centralized polity was creating a pan-Mediterranean culture that had a lot to do with which works ended up preserved, Sparta was pretty much a theme park shell of its former self, and its former self is probably not so amazing anyway. In particular, their unusually thorough and unbalanced slave economy is overlooked when lionizing them, and their military record is rather underwhelming, possibly due to the incredible inefficiencies, even by ancient Greek standards, of said slave economy.