This uses the pushshift.io dataset. The dataset is collected by an independent non-Reddit inc user @jasonbaumgartne on twitter. Huge props to him for collecting it. Anti props to the authors for not giving him a better acknowledgement in the paper.
They could have at least thrown it into the references, although I doubt citation count would matter in this case.
There's a trend right now where researchers and reviewers have no respect for data, even as research uses more and more data. They don't think of citing the data they use any more than citing the air they breathe. This leads to lots of research built on bad data, as producing good data is not rewarded.
I've seen my own data set become a footnote, appear as the only uncited source on a slide, and be attributed to other people who worked on a related project but had nothing to do with producing the data.