It seems to work because it's not doing anything, you can remove this line as the constant is no more supported by the new scintilla version and has been removed.
I recently had a project give the same error half way through. A cheerful silicon friend has been helping me and while it was struggling to open files in Scintilla it 'did something' and suddenly I had the dreaded Constant not found: #SCI_SETLEXER. I had to put #SCI_SETLEXER = 4001 into an include file, but I can't say it's removal was the cause. Otherwise the GoScintilla v3 versions are working fine for me(win10/64), in fact the project started with one of their folding examples.