You missed
my point. In multiple ways. I already accounted for remotely fetched JS in my previous comment.
It is relatively easy to find JavaScript execution points (there are only so many ways to parse and execute text string from a server in JavaScript—eval, new Function, script element, on... DOM attributes, ...), and thus it is quite easy guarantee that an application does not intentionally execute remote code (intentional is what is inside the control of the source code—an image decoding bug in the browser causing code execution is outside the scope here).
A local deployment (with proper protection of the deployment) guarantees that an application remains static, and cannot be changed arbitrarily for malicious intent. Combined with a relatively easy inspection for intentional remote code execution, you can conclude that there is no direct way for the application to turn malicious.
If you do not inspect the source, a local deployment still reduces the chance of a malicious modification being possible from "100% guaranteed" to "maybe, if the application is written in a specific way, or if there is an unintentional code execution bug somewhere in the browser".
You will of course never be able to reduce the chance of any application turning malicious to 0 without full verification of the application and platform it runs on (including all other applications running with permissions to interfere).