Not at all the same. The Greek had the field advantage since they were defending the pass of Thermopylae (quite different from Onigashima), as you said they lost and, more importantly, they weren't 300; that only was the number of Spartans, but the whole Greek force amounted from 5.000 to 11.000 depending on the source (most likely around 7.000 men).
Anyways, real examples of battles where a significantly smaller force ended up winning have happened. But it's pointless to discuss it because this is a fictional conflict; if anything, I guess Oda took inspiration from the Battle of Okehazama (especially because Oda Nobunaga, a likely inspiration behind Oden, was involved there), where around 2.500 samurai defeated the 25.000 army of a warlord from that time by attacking his base and taking him by surprise during a celebration (much like the Onigashima raid).
So Kaidou's 30.000 men losing to a raid force of 5.400 isn't that fartetched considering what Nobunaga achieved. Being outnumbered isn't a defeat guarantee.