Java bytecode and the other technologies you mentioned solve the problem of distributing a program so that it can run unmodified on various platforms.

Reproduceable builds are not about software distribution, they are about compilation. The Arch Linux maintainers would like to be able to verify that the Java bytecode that they are sending to their users was indeed built from the Java source files that they were expected to be built from. With reproduceable builds anyone can double check that the Java bytecode that is being distributed is the correct one by recompiling it on their own machine. Without reproduceable builds you need to trust that the person who compiled the file didn't do anything malicious (either intentionally or unintentionally)