I think Bruce is onto something here:
It seems to me that browsers could do more to protect their users. Browsers are, after all, user agents that protect the visitor from pop-ups, malicious sites, autoplaying videos and other denizens of the underworld. They should also protect users against nausea and migraines, regardless of whether the developer thought to (or had the tools available to).
So, I propose that browsers should never respect
scroll-behavior: smooth; if a user prefers reduced motion, regardless of whether a developer has set the media query.