there is absolutely NO way a packet with an internal IP can get anywhere on the public Internet
That very much depends on your definition of "public internet."
My service provider doesn't properly filter RFC1918 space at their border. From my home on the west coast, I can hit 10.0.0.0/8 devices on the east coast in a separate AS.
Is a reachable printer web interface 3000 miles away in New York on the public internet? The "Internet" is a very tough entity to define succinctly, but I'd argue yes.