Form prefill upgrade - only works when click tracked links in email

[Renee]

Sorry I would like to implement form prefill function to my work. However, when I read this article and feel confused about the last condition. Can someone help explain, thank you. 

https://nation.marketo.com/t5/knowledgebase/form-pre-fill-feature-upgrade/ta-p/251652

It's strange to me if we just copy the Marketo tracked link externally, for example, to facebook posts, in that case, the link is not the unique one, it's like everyone see the link will be directed to the same Marketo LP, how Marketo know who clicks the URL....

I feel like the situation is just like an email with a mkt_tok tracked link is forwarded to another person that may already be cookied as a known person in your database. The tracked email shared with everyone will not match the people who already cookied as known person