I wish that I had been confused, it was an appalling thing to find. Basically, it was an interaction between the template system (https://docs.sencha.com/extjs/6.5.0/modern/src/Template.js.h...) and their dynamic class loader (https://docs.sencha.com/extjs/6.5.0/modern/src/Loader.js.htm...). The template system used regexes to look for JS expressions to evaluate inside HTML templates and then tried to figure out how to supply them with values. I had some /very/ complicated templates and IIRC it picked up on some commented out bits and ended up triggering the loader's dependency resolution. Again, I'm not saying it was running like a babel-style parser in the browser - it was just regexing for certain expressions and dynamically loading classes based on the results.
I saw a similar behavior in their binding formulas, I noticed the formula method would never get called unless you had used a "get" method inside of it. This may have been documented, not necessarily a bug - but it made me think they were scanning the source code at run time to figure out which events to observe.