Well i think you underestimate what algorithms can piece together. And i don’t think it is necessary to know everything about someone. Even if you’re missing half of the picture… A few key facts may be enough to manipulate someone or gently push them into a direction that is more aligned to your goals as a company (for example). Information can be linked after the fact. And - we’re getting a bit philosophical here - You’re kind of the sum of your parts, your history, behaviours and different interests. No single part defines you but still they’re part of you and of what you are. If I can get access to some part of you like your literacy, what kind of media you consume to make up your mind. What kind of people you’re going to meet on social media. I’m starting to affect a part of what is ‘You’ and it also affects you as an entity.
I’m glad you value privacy. I’m not exactly sure what those algorithms do. But there are cookies and there is browser fingerprinting. And it works pretty well. If you use two accounts and use the same device, they can most likely tell by your browser fingerprint and they already know they both belong to you. And even if you’re using seperate devices. If you’re using a residential internet connection, it’s the same IP address for both devices. This is probably also evaluated, because they store that information for the advertisers, because being in close geographical proximity is important for some metrics.