You'd probably need more than 51% to attack Liquid.

Pegging in BTC to Liquid requires 102 confirmations. Even if an attacker had 51% of the hashrate it'd probably be insanely difficult to cause a 102 block reorg.

You'd probably need way more than that, but I'm not sure how much. Also Liquid could just increase the amount of confirmations required more and more if a potential 51% attack is underway