No - it is any examinable syllabus not just ones you are studying or even subjects taught at your school. The syllabus clearly says 'must not overlap significantly with the examinable content of any other School Certificate or Higher School Certificate course.'
Ask yourself - does doing a research project on Hirohito 'significantly overlap with the examinable content of the HSC Modern History course where Hirohito is an option' and your answer is "Yes" so you can't do Hirohito for your project.
What your school is allowing you to do is unethical at least - sorry - but they are giving you an unfair advantage as you could decide to do the Hirohito question in the exam rather than Ho.
It is for reasons like this that I would love to see the projects externally marked so that teachers aren't allowing their students to not abide by the syllabus set by the BOS.
Good luck to you that you have a school not following the rules.
If you think about it - why did it state 'School Certificate syllabus' when obviously students wouldn't be studying that content for the HSC?