The point is that you have access to a 100Mb/s connection, and your access to that connection is unlimited. It doesn't become a 10Mb/s connection at some point, and your access isn't cut off - there are no limits on your access.

Of course there are practical limits as you can't make your 100Mb/s connection into a gigabit one (ignoring that you can buy burstable in a datacenter, etc, etc).

Where unlimited falls down is when it refers to a endlessly consumable resource, like storage.