Indeed they are, at least until the late game when they start using reasonably large stacks (but obviously you'll have more than 30k by then).
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To everyone else: It's not about luck, or being really good at the game, or having other countries fighting them, etc. The golden horde uses sub-30k stacks. Let them assault a province to drain the morale from their infantry (so that only their cavalry are doing anything) then wipe the stack. Repeat. Same way you can easily do stuff like, say, beat Bohemia+Hungary+TO in 1399 with Poland on 25kish men.