I have heard this before, but I have several reasons to think it is not going to be a problem above what already happens:
1. AI lets you generate an enormous number of lies, but what is really dangerous is one well-placed lie within a trusted stream of many truths. CNN will retain a power to mislead far in excess of Twitter bots.
2. Democracy averages over everyone's confusion, which means lies are only dangerous when large numbers of people believe them at the same time. Hordes of bots generating spurious lies won't move a democracy in any specific direction, but again, mass media will retain its power to mislead everyone at once in the same, effectual, direction.
3. People have never respected the veracity of random tweets. In the same way that trust in mainstream media outlets is reaching record lows due to their consistently biased reporting (they might not all have the same bias, but I can't think of any I'd consider free of bias), everyone will learn to adjust their incredulity to match the true quality of random tweets.
4. Companies like Twitter and Google are known to be shaping their results and algorithms according to their own "political views" (broadly construed) so at worst this would represent a partial shift of power from the old masters to new masters quite like them (social media companies). In many ways trimming the front page to reflect editorial opinion is echoed in the way Twitter trims its feeds to reflect their own editorial opinions.
All taken together, it seems like the media is afraid that equally large companies with similar business models (content, attention, advertising) might end up eclipsing them. The same old model where the TV station is afraid to upset its advertisers, thereby giving a voice to business interests, is well-recorded in the recent history of YouTube. Not so much will change, although seeing it in its old and newer forms might shed light on how it works.