Ah, I probably should have used a TL;DR tag - my original comment is just a really condensed summary of the awesome but super detailed article, which even shows

Ah, I probably should have used a TL;DR tag - my original comment is just a really condensed summary of the awesome but super detailed article, which even shows and explains the Struts code in question, example requests and responses, stack traces from the exception, and the exception logs. Even in the comments are some logs that look like evidence of an actual in-the-wild RCE attempt! So maybe check there for more explanation... all credit for the research and investigation goes to the article's author. It was really a good 'deep dive' into the internals of Struts to see how such a subtle bug could be turned into RCE.