I'm not surprised. The border areas are not heavily populated. China doesn't really allow immigration, and Hinduism is not one of the 5 allowed religions in China. There is apparently a small population of Hindus in Hong Kong:
Also to add to Quizmaster's comment, Mao Zedong's party in the mid 1900's put religion under government control and banned most religions and they ran campaigns to stamp out religion in China.
Theistic religions aren't big at all in China and modern China is one of the most atheistic countries. They almost exclusively practice spiritual, humanist or folk religions (Taoism/Confucianism etc) rather than theism of any kind.
(I'm assuming your comment is in reference to Buddhists in Tibet, not Hindus?) Given the mass murders and mass exodus circa 1959, I'm guessing that the remaining Tibetan Buddhist population is quite motivated to severely underreport themselves. Even if they weren't, I'm not sure China would feel it particularly advantageous to accurately report those numbers either.
They didn't here, or anywhere. Google the Sykes–Picot line. Or just look at this map from Wait But Why: https://149909199.v2.pressablecdn.com/wp-content/uploads/2014/09/border-problem-1024x785.png
Pretty small population overall. Hinduism does have a narrow plurality among major religions, but the country's small population is pretty religiously fragmented. For Guyana to make this list it would have to be around 50% or more Hindu.
https://en.wikipedia.org/wiki/Hinduism_in_China
Theistic religions aren't big at all in China and modern China is one of the most atheistic countries. They almost exclusively practice spiritual, humanist or folk religions (Taoism/Confucianism etc) rather than theism of any kind.
https://www.jetpunk.com/groups/invite/mLtD8LKy