It is a separate, already independent nation.
A more accurate analogy would be like saying the US claiming sovereignty over Canada would be a US / Canada issue and the rest of the world should not be involved. That’s not how it works.
I’m all for the people of Taiwan to determine their status for themselves, but I don’t think the people of China should get a vote on the matter.
Canada and the United States both claim to be nations that broke away from English rule. If we claim that common heritage means that Canada is rightfully part of the United States territory (in that we are the successor to English rule in the colonies) would other nations have the right to complain about that? Sure, they claim to be a separate nation, with a separate history of breaking away from English rule, but…
As long as we claim that it was always a part of the English colonies, then no other countries can complain if we claim their territory as our own?
That’s a neat trick.