Spartan panoply at Thermopylae

[JoshoB]

...  muscled cuirasses, as mentioned earlier, were not used until later in the fifth century BC.

Could you elaborate?  I have usually read it claimed that the muscled cuirass dates to the late Archaic, though I won't claim to have scholarly citations for that.

Sure. Eero Jarva discusses the relevant material in depth in his Archaiologia on Archaic Greek Body Armour (1995), where the muscled cuirass corresponds to his type III (pages 30-32). The earliest evidence is a specimen from Olympia that excavators believed date to the early years of the fifth century BC (always difficult), and the earliest depictions in Attic vase-paintings date from around 480 BC or a little later.

Jarva suggests they're a feature of the Early Classical period rather than strictly Archaic, and they certainly don't seem to be as popular as the bell-shaped cuirass was or the linen corslet became from the later sixth century BC onwards. By the time of Xenophon (On Horsemanship 12.1–5), at least, muscled cuirasses seem to have been used largely by cavalrymen rather than infantry.

It's not entirely impossible that someone at Thermopylae used a muscled cuirass like this, just unlikely in my opinion.