Has only been invoked once before

"According to Steven Freeland, dean of law at Western Sydney University, the 1972 Convention on International Liability for Damage Caused by Space Objects states that the launching state is liable to pay for any damage caused. In this case, that means China.

That convention has only been invoked once before, when the Soviet Union’s Cosmos 954 crashed in 1978. That was a nuclear-powered satellite and it showered nuclear waste over Canada. Canada billed the USSR C$6m and it eventually paid C$3m." (The Guardian.)

Why is the present perfect "has only been invoked once before" used (in the context above) when the action of invoking the convention was carried out in the past, (i.e. Canada billed the USSR and was paid after that convention was invoked)?