RDW is not a sensitive/specific enough measure to distinguish between IDA and thalassemia. This has shown up as a key piece of information in NBME forms in which a patient with thalassemia has an elevated RDW (specifically FM form 4, for which I have added a tag).
Some may use a cutoff of 21%, which has sufficient sensitivity/specificity.
There is a very similar card in the overhaul deck:
Anki id: 1516822273369
This information is corroborated by primary literature and also the Amboss article below:
https://next.amboss.com/us/article/tT0Xs2?q=thalassemia
Van Vranken M. Evaluation of microcytosis.. Am Fam Physician. 2010; 82(9): p.1117-22. pmid: 21121557.
https://pmc.ncbi.nlm.nih.gov/articles/PMC6151350/