I took it to mean that the opposition is occurring based on individual positions, rather than a single united entity. Not all white voters were opposed.
The distinction between members of a group acting collectively as a group or individually within the group is one that exists in the UK, but that is rare at best in America. It's clear that "not all white voters" were opposed.
A majority was ("were") opposed. This is explicit by the words themselves, notwithstanding your choice of conjugation. "All" is not distinguished from "majority" by the use of is/are, it's distinguished by the use of "majority" rather than "all."
Your first link doesn't say anything about the UK/US distinction (and its example sentence -- "a majority of voters are unhappy" -- is somewhat unpersuasive, insofar as people frequently incorrectly make their verbs agree with the object of the preposition rather than the subject; its "are"
should agree with "majority" but we can't rely on this example unless we know that the writer didn't mistakenly intend to agree with "voters" instead) and your second one says exactly that: "Thus BrE allows a distinction between (a) and (b) below, while (b) would sound more awkward in AmE ... Thus, AmE speakers tend to avoid sentences like (b) and to rephrase them..." In support of the opposing view it only cites a now non-existent page on the CUNY writing center website (a towering authority if there ever was one) that basically just prescribes the British rule (which the page acknowledges further down "would sound more awkward" to Americans).
That isn't to say that you're completely and 100% wrong, just that it's a pretty dumb thing to call out in the first place. "Virtually always is not always" is technically true but is an underwhelming argument when you're the one nitpicking the Star's grammar -- because the writer used the construction that is virtually always (seems like you are tacitly conceding this point), but not necessarily
always always, used in America?
This is a pretty bad conversation to have in a streetcar thread, probably should have just kept my mouth shut.
(edited for a little further clarification)