Since EOS launches its coin on Ethereum platform, they own ETH in billions of USD value.
They can play games with it and influence the price of ETH if they want.
What do you think they will do with this amount of ETH? Is it not dangerous situation for the ETH?
If you are hinting that they will start lowering the price of a competitor, for their own promotion, then I doubt that they are going to do it. At last, in percentage terms, relative to the total amount of the Ethereum, they do not have much. I believe that this is investments, and they will spend the on the promotion of their project, and not on the drop down of prices.