If you read it, you'll find it explicitly states they could not establish a causal relationship between investors and increased prices. (i.e. it says exactly what I stated).
They do find a correlation (which is heavily referenced in the news and misinterpreted), which would be expected. Investors who study the market and are investing professionally, are more likely than the average, and definitely more likely than below-average, to invest in neighbourhoods which turn out to see more price increases than neighbourhoods that investors chose not to invest in. That's of course because it's their job and their expertise. It doesn't mean they're causing the price increases.
It's a bit like saying there's a correlation in prices between prices of stocks of companies that professional investors choose to invest in or not. That's obvious, professional investors seek returns, and are expected to invest in such companies. But the investors aren't causing the tech stocks to become valuable. They simply are, by way of their profits. Google (once established) isn't high-value because investors choose to buy its shares. Investors detect the value and pursue to own it. Real estate is no different. Which is why the study found a correlation, but explicitly did not establish a causal relationship.