I don't think so, the sophomore in COMP 101 isn't that valuable to SO because they add no knowledge to the ecosystem, they'll come from Google, read the answer and leave. But the expert Java dev with 15 years of experience who posts on Usenet forums is someone who may have very specific preferences and will entrench themselves in a platform.
I'd assume these news also apply to the properties on stackexchange that have a lot of non-tech content. As for the rest they probably want them to create accounts, that'd be one way to do it.
It's beta, the average user isn't supposed to see it yet:
"We want to share it with our community, especially our power users, and gather feedback so we can improve, iterate, and expand Dark Mode in the future."
One of the beta is "System setting" which pulls the default from the browsers "prefers-color-scheme" value (which by default pulls it's value from the system scheme). When it's ready to roll out fully this seems to be the likely path they are planning.