Wait... no! That's not how you do it!

yellow is justice, and is a real soul in undertale

I think he's saying t's meant to be upside-down because that's how it works in the Mettaton fight, but the Alphys variant and the justice soul are diffeent so it shouldn't be upside-down.

The most basic description of the difference:
Justice soul: The soul that belongs to a dead sheriff boy.
Alphys' artificial soul: Let's put some bullets in that empty gun.