It is funny, in a sad way, the Pro documentation is so vague because the ArcMap documentation has always been quite clear. For example, the description for ArcGIS Data Reviewer 10.5.1 Patch 3 for (Desktop | Server) states, "This patch is cumulative of all previously released Patches and Hot Fixes for ArcGIS Data Reviewer 10.5.1."
Back to your question. Can I answer it with certainty, no. Since Esri doesn't state which files are updated to which version with any patches, and they don't state that 2.1.2 is cumulative, I can't say unequivocally that the 2.1.1 fixes are in 2.1.2. That said, I am able to enter minus signs for long and short integer fields for attribute tables, which was an issue (BUG-000111161) addressed in the first patch. So empirically, or maybe anecdotally, the 2.1.1 fixes are rolled into 2.1.2.