The jury is still out on whether or not it is related to SUDI. There is a good paper published a few years ago: Anna S. Pease, Peter J. Fleming, Fern R. Hauck, Rachel Y. Moon, Rosemary S.C. Horne, Monique P. L’Hoir, Anne-Louise Ponsonby and Peter S. Blair. Swaddling and the Risk of Sudden Infant Death Syndrome: A Meta-analysis. Pediatrics June 2016, 137 (6) e20153275; DOI: https://doi.org/10.1542/peds.2015-3275
Basically it found that there was a non-statistically significant increased risk of SIDS from swaddling. The risk was high for babies placed prone and moderate risk for on the side compared with babies on the back. This makes sense from an airway obstruction perspective.
Infants in sleeping sacks or bags are at lower risk of SUDI. These babies are not swaddled.