Agreed: India and Thailand, or India and Korea, or China and Laos, et cetera, are very different. Like Ghanaians and South Africans. But to someone looking from the outside through a racist lens (not you, obviously, the shooter and anti-Asians), that diversity gets erased.
So, pan-Asian American solidarity is a thing. Americans of all sorts of different Asian national ancestry and also Pacific Islander ancestry identify as a group, diverse as it is, which has had similar experiences as immigrants to America.
Anyway, tl/dr : It is valid for Indian Americans to call themselves Asian Americans and be concerned about the treatment of Asian Americans as a class.