I believe The Bible 100%.
With that said, was Goliath a Philistine. He "went out from the camp of the Philistine army." David called him a Philistine, but that does not necessarily mean any more than he was a member of the Philistine army. Was Goliath a Philistine or a Philistine mercenary. Both are possible. The answer does not contradict Scripture either way.