What does that mean?

Hi everyone

I'd just like to know something that may be blindingly obvious to most...Sorry if it is. I see on GXL shares fairly frequently notices such as "NAB becoming major shareholder", the document attached says that it is NOT price sensitive. After this, I see people refer to this as an "attempt at a short" which would indicate that becoming a substantial shareholder COULD effect the price of the stock. In simple terms is it possible that say for example an entity bought $10m of GXL shares, could this dramatically impact the share price and if so, doesn't this lend itself to major insto's being able to manipulate the stock to their own gains?

Cheers for help in advance.