I do not believe the claim. At the moment there is a war between reddit users and reddit managment. The managment is looking to IPO and so is trying to demonstrate its ability to wrench profits. The users are reacting through demonstrations of thier power to impact shareholder value. This hacking claim falls squarely into the later: it threatens shareholder value by introducing uncertainty. I will need to see solid evidence to be convinced.
Redditors are being used as pawns by the third party devs in their fight to maintain their free loading existence. Which was easy, social media is good at riling up people for made up reasons.
Reddit has several hundred million users, and even more anonymous visitors. The two biggest third party apps have 1.5M users combined.
