They didn’t just detect mRNA. They specifically screened for the mRNA used by each vaccine:
“.. we developed specific RT-qPCR-based assays to detect each mRNA vaccine and screened lymph nodes, liver, spleen, and myocardium from recently vaccinated deceased patients.”
https://www.nature.com/articles/s41541-023-00742-7
The authors of this article are too stupid to mention that key fact.