That smells like bullshit.

Asymmetric warfare ( or guerilla warfare) has been successful time and again since the Napoleonic wars, regardless of how overwhelming the enemy was. And Napoleon himself won multiple times against the odds ( so much so the Coalition made a deal to only attack when he isn't in command).

And there's the usual incompetence, bad luck, or tactical/strategic brilliance.

Just in the last century we have the Russians ( in Russo-Japanese war, WWI, Winter war, WWII), Austro-Hungarians, Italians, Ottomans, Americans ( multiple times), Arabs ( against Israel) fail miserably against a theoretically inferior in terms of size and weaponry enemy.