Is the SLS tank significantly thicker than the ET, or is it just an unprecedented thickness for friction stir welding?
SLWT introduced Al-Li alloys, orthogrid structures, and (IIRC) friction stir welding.
Friction stir welding was used for SLWT beginning with ET-134, flown on STS-130. https://www.nasa.gov/centers/marshall/news/news/releases/2010/10-010.html