Well honey is 80% sugar by weight, lemons are 90% water so I'd imagine that the liquid that comes out at the end is going to be 30-40% lemony sugar water. The photos I've seen online suggest there's about the same amount of honey as lemons?
A teaspoon of sugar is 4g of carbs, so a teaspoon of your liquid is maybe 1.5-2g of carbs. How many teaspoons would you have in a day? The fermentation process might lower that figure somewhat, I'm not sure. Unless you're having a lot I doubt it would kick you out of ketosis.