I don't understand P.Olivia optimal build

Her attack is only 33 (38 with L&D3) with Poison dagger+, which is too low even with the 1.5x multiplier against infantry. Also, L&D should never be used on a dancer for obvious reasons.

I think the windsweep+phantom seal combo is just to prevent counterattacks and doubles.

Olivia is a support unit and her build is meant to accentuate that. With Windsweep, when she doesn't dance she can attack the opponent for free if they're physical and that both debuffs her opponent and heals her allies. It's 3 support units all in one.

It's more focused around being a support unit, which other than her access to Poison Dagger, is one of best things about being a dagger dancer.
The ability to mix and match between debuff, healing, and dancing is quite the combo, and with Windsweep with her solid speed tier, she won't be afraid of retaliation from common DC units on initiation.

Finally, this is mainly directed at Cosmo, I just want to mention that 52-57 (Fury and LnD respectively) attack against infantry is definitely not too low, as a matter of fact, it's some of the highest in the game. Put that together with 40-42 speed and she becomes quite the offensive powerhouse for a dedicated dancer.

I believe the idea it revolves around is to build on the idea that she can act as both a dancer and as a chip damage debuffer while staying relatively safe from DC and whatnot.

I wouldn't say it's the most optimal build by far as there are plenty of different ways you can utilise her. I like WoM with BoL as when she has access to teleport to an ally, you have the option of dancing or hitting a unit to debuff and healing your ally.

Most of the site's optimal build actually feels pretty reasonable. The one thing that feels very questionable to me is the use of Aether, which I can only assume is for goosing her arena weighting. It seems like Miracle might be a more interesting choice and that the alternate Luna would be more useful.