Clearly some Hindus believe that Buddha is indeed the Kalki avatar. Otherwise this thread would have no reason to exist. (Update: clearly I goofed here. Sorry. Disregard the previous statament.)
Which is the correct belief? I have no idea. Or rather, I don't think that there is any objective way of telling.
Instead, the reasonable stance to pursue is IMO to accept that different groups and schools of thought and belief will often use the same or similar words with sharply different meanings. More than that, the meanings themselves are also influenced by the general doctrine and beliefs. Therefore it is rarely very clarifying to pinch simple statements without much context.
Regarding this specific question, I would propose that the claim that Buddha is the Kalki Avatar of Vishnu is indeed at odds with what is generally understood to be Buddhist teachings. But those were not written to necessarily conform to Hindu doctrinary concepts and expectations; certainly not to those of all Sampradayas. Nor should we expect otherwise, since even by a purely Hindu perspective Buddhism is, after all, Nastika and therefore divergent from more mainstream, Astika lines of thought within Hinduism.
The way I see it, the validity of contrasting Dharmic teachings is to be gauged mainly by the ethical results of those teachings. There will be (and is) a myriad ways of wording Dharmic / ethical teachings and at least as many doctrinary models to accompany them. Many of those models are indeed at odds with each other, sometimes very directly.
That is not really a problem as long as each group takes proper responsibility for its own models, concepts and teachings and proper care to detect and correct mistakes of behavior and motivation.
Ultimately, the concepts hold no wisdom in and of themselves. It is for practicioners to use them wisely and accept the effort, dangers and merit that come from that use.