Every single Sonic 1 disassembly I can find doesn't seem to include split.bat. You know, the thing that you need to actually disassemble the game. Why is this?
You're probably looking at a tutorial to set up a disassembly that's outdated. Everything should be there for you already. Just modify what you need to, run build.bat and you should be good to go. Questions like these should go to the basic questions and answers thread, by the way.
Even though it's already 'disassembled', I still honestly think that 'split.bat' is useful, and I find it dumb to not include it. I always used 'split.bat' to not only just disassemble the game, but to also help me restore certain files for my hack, in case those files in my source code became corrupted.