From whatever I could read over internet, they are pretty similar as they both aim to perform stream and batch processing with similar models.
Vice president of Data Torrent stated the following:
"Apex and Flink are conceptually similar, according to Thomas Weise, DataTorrent’s vice president of Apex (not to be confused about the serverless computing framework of the same name). Both Apex and Flink can do batch processing, but are more focused on streaming. And though he concedes it might make sense to merge the two projects, he doesn’t see that happening, primarily because of their different roots."
Also, both aim to follow the Google dataflow model and check out most of the same things on this capability matrix: https://beam.apache.org/documentation/runners/capability-matrix/
If they are so similar then why they are still both Apache top level projects, f they are different, what makes them concretely different from one another?