Lithium is not present in its metallic form in rechargeable Lithium Ion batteries.
Even if hydrogen were created you would have to create quite a quantity to create concentration that are high enough to be a danger except form maybe a small flame over the water where the gas is escaping. Now unless you light the bubbles coming up it's likely that it will just dissipate and mix with cabin air. Which is harmless unless you somehow produce big quantities.
As the video above states the problem are the flammable electrolytes contained.