The person you are responding to is probably talking about modern humans. Where groups separated by geography and with a different language have definitely exis

The person you are responding to is probably talking about modern humans. Where groups separated by geography and with a different language have definitely existed. Though I would argue that at least in the modern times, communication and transport mean that no identified group of humans is actually totally isolated anymore from the other humans, which makes it more difficult to argue that they can be different species on this ground.