I'd say an important part of this calculator's interaction model is doing something, getting a result, then doing something else to that result. That's not too bad in the regular Python interpreter either.
For example, in Python:
>>> 5
5
>>> 4 + _
9
>>> 2 * _
18
In Stacker:
>>> 5
[5]
>>> 4 +
[9]
>>> 2 *
[18]
Does Hy have something like the Python interpreter's _
?
Just note that the comment was inaccurate, in that their weird encryption is indeed open source at least.