Yes, mangrove snakes DO eat other snakes, as I found out recently from a YouTube Short. How their mild venom affects the the snakes is unknown to me since all the videos I've seen of them eating snakes are when the prey is already dead.

As for this sock-eating one, they'll gladly eat sock snakes alive as long as they are smaller than it, no venom necessary. Sure the sock snake will fight back and the venom is unable to even knock it out due to the mangrove snake being no sock snake, but when it goes in nicely either way, who even needs venom?