TheMasterCaver is correct that the programming language does not matter, so long as the software adheres to the protocol. As long as they get data in the expected format and with the correct timings, language is irrelevant.
TheMasterCaver may be correct on how hard it is to deal directly with the Java bytecode. indicates the MCP was used as part of the decompiling process. I admit I am not fully versed in the process, I've not tried it myself. Which is easier, decompiling Minecraft Java using the MCP or decompiling and making the same kinds of changes to the Minecraft Bedrock version? And would the answer to this question be significant towards a reluctance on the part of Mojang to include Java in the Better Together update?
As I said earlier, this is entirely incorrect - let's see, what OS are you running? Windows I presume? Most Web sites use some variant of Linux, which has a very different filesystem from Windows - so how can we send files between computers? Because it is only the way they handle network traffic and interpret the data that matters - just as I can use a program coded in C++ to open a world created in Java: