i should also note that a very common reason for a dedicated hosting company's server to go wild on upstream bandwidth is something misconfigured or compromised, and being used maliciously by some external 3rd party. not actual bandwidth usage from the paying customer. this is mentioned in a nicer way in the email.
they probably could have just cut them off arbitrarily. nothing obligates them to keep a customer.
this is actually really nice on the part of the hosting ISP in my opinion.
If you have a sustained near 800 Mbps of outbound traffic you should be doing it from something much more serious than an absolute bottom of the barrel lowest cost no-SLA dedicated server.
this is "unlimited" in the same way that an all you can eat buffet restaurant is unlimited, with some caveats.