Interesting paper.
The abstract is aligned with some of the principles on this summarized by EMA in an open access paper (equally citing the original Casado work):
Formation of N-Nitrosamine Drug Substance Related Impurities in Medicines: A Regulatory Perspective on Risk Factors and Mitigation Strategies | Organic Process Research & Development (acs.org)
Historically we had to assess when and if formaldehyde (ppm level LOD/LOQ) testing on medicines was critical to exclude this possible (dominant) root cause (as described in the EMA paper) in nitrosamine pre-testing and post-testing, anyone having to do the same will benefit from buying the paper.