There's no technical limitation preventing software development in Java verses C. The only major advantage is execution speed. Moreover, as JEB is directed towards Java programs (android APKs) writing it in Java makes sense.
However, this question is not really about reverse engineering.
IDA results look pretty good. I believe that the signature of main comes from any kind of FLIRT or other function recognitions - It detects that the function is main, and therefore gives it the default main signature. it looks like foo disassembled well. You can configure yourself the signature by pressing y on the function call. You must understand that the ...
Apparently yes most of JEB code base seems to be in Java. You can look at the jeb*.jar files inside your install folders. They're huge, but that will give you an indication: thousands of big javaclass files in there, very few native so files.