Converting ANTLR AST to Java bytecode using ASM

Sure it is. Everything that introduces another level of interpretation into the computing stack is a virtual machine that a programmer can target. It may not become a very complete or successful one unless you put insane amounts of effort into it, but your claim to the technical term is valid.

"Machine code" is a more restricted term. Surprisingly, the convention is not to call code that runs on a virtual machine "machine code", but "byte code". "Machine code" is usually understood to mean code to be run on a physical microprocessor, i.e. a stream of its opcodes. Anything below that is micro code, and anything above is either "byte code" or just "code" (Java code, Python code, etc.)

The logic is that JVM bytecode is a lot simpler than Java source code.

Compilers can be thought of, at a highly abstract level, as having three basic parts: parsing, semantic analysis, and code generation.

Parsing consists of reading the code and turning it into a tree representation inside the compiler's memory. Semantic analysis is the part where it analyzes this tree, figures out what it means, and simplifies all the high-level constructs down to lower-level ones. And code generation takes the simplified tree and writes it out into a flat output.

With a bytecode file, the parsing phase is greatly simplified, since it's written in the same flat byte stream format that the JIT uses, rather than a recursive (tree-structured) source language. Also, a lot of the heavy lifting of the semantic analysis has already been performed by the Java (or other language) compiler. So all it has to do is stream-read the code, do minimal parsing and minimal semantic analysis, and then perform code generation.

This makes the task the JIT has to perform a lot simpler, and therefore a lot faster to execute, while still preserving the high-level metadata and semantic information that makes it possible to theoretically write single-source, cross-platform code.