[quitit]

You are right: borders don't matter - in reality the disease is eradicated well beyond the borders. Also keep in mind that many animals aren't great travellers, especially so if infected by rabies.

Claiming eradication is based on continued evidence from robust and internationally recognised monitoring and eradication programmes.

See the "Methods" and the footnotes sections from this CDC page: https://www.cdc.gov/rabies/resources/countries-risk.html