The GP, along with many other people, are trying to get software professionals to standardize on a definition of simple that means something like "composed of a single element; not compound" or "easy to reason about".

Under such a standard, it makes no sense to talk about "simple to use" vs "simple to understand". You can talk about "easy to use" vs "easy to understand". The latter might mean "simple". But "simple" never means the former. And you never say it's "simple to X" for any value of X.

Under such a standard. The degree to which people are converging on this standard in actuality is not clear to me.