Is it true that MPS was made not to create a programming language, but to create syntax for an already created language (highlight)?
No, that is not true. You can make any language (generic programming language or domain specific language) in MPS that you could also make with your own custom parser/compiler, and more. Because MPS supports multiple notations (provided you install some plugins like the diagram, tables, or math notation plugins), you can make languages with directly editable concrete syntaxes, which are not only textual, but for example a diagram, a math formula, a table, a tree, or a mix of various notations.