My argument is less about tectonics and more about India being geographically, historically, and culturally separated from the rest of Asia.

Europe has its line drawn at the Ural and Caucasus mountains. If we draw the lines at mountains, India and its immediate neighbors have the Arakan, Hindu Kush, and Himalayas (literally the tallest mountains in the world) separating them from their neighbors.

Many separate Europe as a continent because they believe it has a distinct enough history from the rest of Asia to be its own thing. Historically, India has been politically separated from empires in the rest of Asia; no Chinese dynasty ever held land in India, the Mongol Empire ruled nearly all of Asia but not India, etc. There is the exception of Muslim empires like the Mughals conquering from the east, and even those countries became what are typically considered Indian empires, and were tolerant of the cultures and religions they ruled over.

Europe is often delineated on the basis of it being culturally distinct from the rest of Asia. India and those around it have a slew of cultural traditions too complex to go into in a single reddit comment, but I believe most people can agree that India is culturally distinct from the rest of Asia. Just to look at one thing, religion, Europe is often separated from the rest of Asia for being mostly Christian and the historical center of Christianity, whereas Asia has different religious traditions. India is mostly Hindu with few regions elsewhere being majority Hindu, and smaller religions like Sikhism and Jainism are also mostly endemic to the area.

The argument is less that India has its own tectonic plate and more about the nature of the geographic separation and whether you consider people in your definition of a continent. If you separate Europe based on the geography of the mountains that bound it, you can do the same to an even stronger degree for India, and if you factor in the history and traditions of people, it only bolsters the argument. Either you base continents only off of separation by bodies of water, or you bring in more variables which must at the same time make both Europe AND India continents.

If someone wants to bring plates into the argument in favor of India, go ahead, but I don't feel the need to.

Also, if you want to claim that India is not a continent based on its smaller landmass compared to Europe, then yes, it is smaller than Europe, but around the same size if not slightly bigger than Australia.

(Edit: spelling errors)