If your opponent has only 1 Island then the only spell they can cast is a cantrip
All their threats get discarded
All their counterspells get discarded
In those 6 turns every time your opponent draws something they can't use, Liliana is providing you advantage and giving you a pool of loyalty to deal with any future threats they happen to play (or half their manabase), in the meantime hopefully you draw a threat you can cast (or a Mishra's Factory).
In the game 2 that was recorded the Pox player doesn't draw any castable threat behind their Liliana for like 4-5 turns but the opponent concedes to SPAWNING POOL
If you don't have Liliana in your deck vs Miracles you don't have any consistent way to deplete the opponent's resources, so when they draw Mentor it's much easier for them to cast it with mana and a spell up, and when this happens you just lose the game instantly
If you have an actual suggested sideboard configuration with his decklist vs miracles that includes cutting Liliana of the Veil I'd be glad to hear it

