I am very tired of the phrase "easy/hard to reason about." It sounds so hand-wavy every time.
I would be fine with "easier to reason about" if they did a comparison study to some other language.
For example, have participants do a few comprehension tasks with Bosque code snippets and also with C++ code snippets. Measure task completion time, task success, and self-reported cognitive load [1]. Then do a semi-structured interview to get some insights into the participants' opinions.
[1] Cognitive Load Measurement as a Means to Advance Cognitive Load Theory https://www.tandfonline.com/doi/abs/10.1207/S15326985EP3801_...
A concrete example of "hard to reason about" is:
High level languages exist to remove the complexity of opcodes of the bare metal running underneath. And that's because assembly code (unless you're well versed in it) is hard to reason about.
As I mentioned in the discussion two days ago, PL researchers throw around these terms but never actually measure them. You can learn a lot from a small user study.