Not OP but there are 2 ways to get it back:
In the sample build he gets it back 2 levels later when he gets Greater Invisibility which achieves pretty much the same result.
If he had gone Half-Elf Drow Heritage he can gain access to Fairy fire and gain perma advantage via that, although he does...
This is my all time favourite build guide and I have played variants of it (Mostly Shadow) many times. In fact I am playing one now and about to get to the sweet level 7 Mark where the DPR is kicking in!
However there is one aspect I would respectfully disagree with and that's Empowered spell...