Well, I agree in general, but that video was clickbaity. Peter Jackson did not put Eowyn in a "combat thong".
That was Frank Frazetta.
So of course she wore a combat thong! Frazetta trumps pretty much every other genre, yes, Tolkien included. Every heroine becomes voluptuous and nearly nude in Frazetta's glorious universe - so the real question is: why would Eowyn not deserve the same transformation as all the other girls? I mean, Frazetta is the master of his art!
(Actually I would say the image in the video gave Eowyn much more sensible combat clothing than almost any other named heroine, so perhaps it's the other way 'round and Frazetta does respect Tolkien?!
)
Plus, his magical transformations aren't reserved for females only. Heck, I bet if Frazetta drew Treebeard he'd be the most oiled up and muscular Ent you'd ever see.