> they don’t have the original data (it is owned by fb) to create the lookalike audience.
they have the original data of their own audience. So they can send it explicitly to FB (instead of FB sucking it from the page's pixel so to speak) for FB to build the lookalike audience which FB would do using the wide FB owned data.
the data they hold on their audience is irrelevant here. eg BB not collecting age and gender, and they don’t have the web history of the user (that fb pixel and thumbs icon enables). the lookalike audience is based on FB profile.
they have the original data of their own audience. So they can send it explicitly to FB (instead of FB sucking it from the page's pixel so to speak) for FB to build the lookalike audience which FB would do using the wide FB owned data.