That's what I would assume too, but one of the slides very specifically claims:

> The presence of an arc=pass generally guarantees a better spam score. Confirmed with ProtonMail. Seems to be the case with Gmail and Outlook as well.

And it doesn't seem like there's anything that would prevent me from adding this to my own emails. I don't think it would turn blatant spam into non-spam, but it seems like it could help something that's already teetering on the line.