I don't see how that link is relevant. Essential in that link simply indicates, as I read it, that the specific way the text of an image logo is rendered is essential to it's function. In other words that you can't simply replace the WASI in the image with the text "WASI." Essential does not mean that is must meet some requirements or be readable. In fact the contrast requirement your presentation cites is explicit in that it doesn't:

https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-cont...

>Logotypes: Text that is part of a logo or brand name has no minimum contrast requirement.