Looking back at what other countries did centuries before is a weak argument for justifying what one does today. The “international community” includes more than a western minority when it comes to the issue of Hong Kong.
When China says "You know what, China? No. We will not break our own agreement. How could you even suggest that? Hong Kong is sovereign" (as the United States recently did in McGirt v. Oklahoma), then we can talk about whataboutism. Until then, that kind of argument is utterly unconvincing.
Always disappointing to see such vitriolic rebuttals to the mere idea of considering a Chinese perspective.