TLDR: They had the dataset lying around from previous (unrelated) research and can't get new data because Facebook removed API access after the Cambridge Analytical Scandal.

"This corpus comprises Facebook pages and, within them, views representing different positions towards Brexit, with some being in favour and others against [10]. It was compiled as part of previous research [10, 11], but ethical approval for new analyses was sought and obtained in 2022 from the University of Edinburgh. We chose to analyse our existing dataset for two reasons. First, examining social media data about Brexit, a high-profile event that has been intensely studied, offers significant opportunities for comparing our findings to existing work whilst contributing to ongoing scholarship on public discourse about political phenomena. Second, in the UK and in many other countries, institutional ethics policies require researchers to acquire data in compliance with platforms’ Terms of Service (ToS) agreements. Like other major platforms including X (formerly Twitter), Instagram and TikTok, Facebook’s ToS state that data must be extracted using its application programming interface (API). Yet following the Cambridge Analytica scandal, Facebook closed its public API, making it challenging to acquire additional data from the platform. "

Quoted from part 2.1 Materials