There was confusion about what volcano had erupted. Only Novarupta erupted early in the 20th century. However, it drained a magma chamber 6 miles away at Katmai forming a Caldera.
Mount Saint Helens and Rainier are 50 miles apart. They are completely different volcanic systems with entirely separate magma chambers.
FWIW, when it comes to stuff like this, I never say never :) Look how long it took'em to figure out what really happened up in Alaska.