Nations have the right to exist and be sovereign. But it’s more likely that he meant „governments“ and he said „nations“ to sound less totalitarian.
No. People have rights, nations do not.
Who has the right to make such a statement and call it „legally binding“ and why?
“Who has the right to make such a statement “ Anyone. “legally binding“ Unless it’s voluntary, it’s no more principled than rape; no matter how much authority you want to defer to, or how many you strawmen you construct.