Yes, you are right, they are technically mostly equivalent. (JSON has simpler primitive types, clear escapes, lists/arrays and maps/dictionaries).
But the point is, there are so many formats out there, and everybody likes to make there own.
If you pick JSON, the discussion ends. It is an RFC standard. If you pick something that looks suspiciously like some (for most people) weird programming language you will get discussions, always.
Dale said so: it is a pragmatic choice.
Now, given the fact that the domain here is Smalltalk anyway, there is something to say for using a Smalltalk based representation.
But then you need to write a clear spec and a non-compiler based parser.
Let us do it.
With the JSON meta data, you could envision other non-Smalltalk tools using it more easily.
Do you want to build our tools in Javascript or other languages? Because we are talking about class definition and smalltalk meta data itself.