They use Google tag manager in their signup page. Translation, Google will know your user name, tie it to your real identity, and will track everything you say.
They'll track everything you share, like, comment, etc. You're helping them build a profile on you. Unless you're completely insulated from all advertising, they will find ways to "nudge" you into making purchases and forming opinions.
Nudging is a slow process that happens on the subconscious level: https://www.convertize.com/what-is-nudge-marketing/
Nudge marketing refers to deliberately manipulating how choices are presented to consumers. Its goal is to influence what consumers choose, either to steer them toward options that the marketer believes are good for them or simply to stimulate purchases and increase sales
Probably a sign.
That's ironic they'd mark their own data collection front as spam
That's a vast oversimplification. Nobody "owns" the internet, it works on nodes and servers. Google does not have a monopoly on this.
There is still a way to be careful online using the right extensions and routing your traffic accordingly.