((Simple: they made a guess. The I assume part was to show that. An intelligent one but mainly a guest. If you remember in C8 Ida made mention of running into Farran/Sejiko. Then seeing him at the battle and the blades showing up, well that was a little to coinsidental so they assume Farren is the one the blades are looking for. If that confuses I'll explain more later.
Also, unless the blade is like a lightsaber you still can kick it away. For instance the hand guard could be safely kicked. Also their armor is pretty strong, and the valkyrie didn't kick the blade with to much force just enough to move it a little)).
Valkyrie: Well, they should reconize you as Sejiko shouldn't they? We have recordings of your transformation. If that doesn't work we can kill you ourselves.
Hirst: You know very little about this dimension blade. You really do need to talk to your two operatives who were here before you.
"Dulce bellum inexpertis."
Official Forum Expert on Norse Mythology
As Odin says in the Hovamal:
"Praise no day 'til evening; no wife 'til on her pyre; no sword 'til tested;
no maid 'til bedded; no ice 'til crossed;
no ale 'til drunk."