This is used in Archeology because humans could travel easily across the same pieces of land. E.g 10.000 years ago humans could cross easily the English channel because it did not exist as the water level was way lower.
There are barriers that isolate parts of Europe and Asia, like desserts and mountains, but very early there were determined humans that crossed them.
[1] https://www.independent.co.uk/news/world/americas/new-eviden...
Always thought a continent was a clearly defined.
Plates with large land masses are the Eurasian, Australian, North American, South American, African, Arabian, Indian, and Antarctic. The Caribbean plate has part of central America and several large islands on it, the Pacific plate has a number of large islands like Hawaii (and part of New Zealand), the Cocos plate has the tip of Baja California, and IIRC the remaining plates have only small islands (Filipino, Juan de Fuca, Nazca, and Scotia plates).
So by a definition of "tectonic plate with large landmass" you'd get at least 8 continents.
[1] https://en.wikipedia.org/wiki/Genetic_history_of_Europe#Gene...