Another possibility is that the spammers have adapted and rolled out the next phase of their plans. After creating a million or three Gmail accounts (aged, so they're all months old, and staggered, so they're not all created at the same time, and with some amount of fake "organic" traffic such that they seem they're owned by real people), the obvious first phase, as a spammer, is to sign up for your own newsletters, and mark those as "not spam" from your three million fake accounts.
As a spammer, you've got to assume Google's going to catch onto your first three million fake accounts and mark both those accounts, the email servers' IPs you use to send, and the email list itself as spam. Consider your first three million gmail account burned by that point.
The less obvious second phase then, using your next three million fake, aged, but-with-fake-email-traffic-to-make-them-seem-real Gmail accounts, is to sign up for not-spam newsletters, but mark those as spam. The wishful-thinking hope is that with enough of those marked as spam, the wider public will see more articles like the linked article, and ever so barely push back on Google, who then have to figure out another way to distinguish signal from noise in a vastly complicated and ML-enchanted algorithm.