Another similar belief would be that Jainism, Buddhism and Sikhism are merely Hindu sects, but I think that would be false.
This is commonly held to be the case by Indian Muslims. (It may be that many Hindus also feel this way, but I don't know that firsthand.) I don't typically express this opinion because it's normally seen as impolite, but I don't think it's wrong. The term "Hinduism" encompasses religious systems sufficiently diverse in themselves that picking out certain strands and labeling them as not Hindu feels arbitrary.
I have a Sikh friend who says this is a common Hindutva trope. But generally the definition of Hinduism is acceptance of the authority of the Vedas.
I am far from an expert on this, but I've been told that this is a belief by both Hindutva types and liberal Hindus though for different reasons. The former dominate today and want to ascribe everything to Hinduism for the obvious reasons. But the latter were more common a few decades back and trace back to 'god by a million names' beliefs.
Historically, Hinduism was not really a religion as we know it and was just a collection of indigenous practices that eventually became unified under an umbrella among religious scholars (but not really commoners). A germ of this umbrella probably started around 700 by Shankara (
https://en.wikipedia.org/wiki/Adi_Shankara) but Hinduism as a religion is arguably only a few centuries old and was formed as a reaction to takeovers by monotheistic Moslems and then Christians.
There are also Hindus that dont follow (or know) the Vedas. Today, most are tribal, but historically they were theological schools that competed with various schools that eventually fell under Hinduism. Whether or not they are 'true Hindus' is a question that the Hindutva and left wing Hindus sometimes agree on, again for different reasons.
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