No, Angkor Wat (or rather the first temples as it is a complex) was built as a Hindu temple by the Khmer Empire's kings in the 12th century and was added onto until the 16th. After Jayavarman VII, the Buddhist presence at the site begin to increase dramatically due to Jayavarman's patronage. In ~1295 Indravarman ascended the throne and made Theravāda the state religion and realigned it with Mahāvihāra doctrine. Hinduism in Cambodia, as a seprate religion from Buddhism, had faded into the past by the 17th century.

In looking into this, I thought perhaps there was some inscription of a South Indian donor or land grant at the site, but there is none. I can find no inscription in the many of the site mentioning any South Indian Donor and while my sources are incomplete, I find it extremely unlikely that a South Indian contributed significantly to building it.

What I did find, while scouring to find the source of the claims was a bunch of Tamil nationalist posts on Facebook and Quora about South Indians building it as well as being the real kings of the Khmer empire, the Śalendras, the Śrīvijayans, and the Majapahits, which is honestly what I expected unfourtunately.

Now here's the weird part about these claims, they're unnecessary. The Indian, especially South Indian influence on Southeast Asia is undeniable. From at least the 3rd century BCE, there was a trade network that connected Cambodia, Indonesia, Odiśa, Tamil Nadu and Laṅkā. Through this Hinduism, Buddhism, writing, and art from India entered Cambodia and the Malay peninsula. All writing in Southeast Asia is based on the Pallava script. In the first century CE, the first major state was established in Southeast Asia, called Funan. Funan was influenced strongly by South India and was Hindu. Ritual practices resembled those of South India and kings deliberate styled themselves in the pattern of Indian rulers.

Thing is, Funan had fallen in the 6th century after centuries of decline. While kings still patterned themselves as Indian well into the Khmer period, the culture had become ingrained into Cambodia and become as Cambodian as Indian. Contact never stopped, but with the exception of Rājarāja Coḷa and his son Rājendra, no South Indian king ever conquered anywhere in Southeast Asia, and even they only conquered part of Sumatra.

In any case, its bull.