Because the very act of storing information in your brain gives it credence, even if you don't go through the extra effort to verify whether it is correct. With repeated exposure it can become a belief, even a belief so strong that you will hold onto it in the face of evidence it is false. This can happen to anyone; it's a fundamental feature of how our memory works.

It works especially well for information that does not affect you personally, because you're less likely to go through the extra effort to verify whether something is true.

If your boss tells you that your salary is $100k/year, you're going to check that on your paycheck!

If your boss tells you that garbage collectors make $100k a year, you might not believe it, but you might also not bother to put in the effort to check. But if you remember it later on, the very act of bringing it out of your memory will reinforce it to you.

And if you don't go specifically check on the accuracy, you're also more likely to rely on social signals like, who is telling me this? Is this a person who I identify with? Does it seem like a person or an institution I should trust?

In this case, you are reading a news article from TechCrunch, a well-known, successful, and long-running news site. Any information you read there comes wrapped in however much trust you associate with TechCrunch. So if a claim is passed on in an article, it comes with some level of default trust.

And even if you don't like TechCrunch, the default level of trust is affected just by the act of reading the article. By default we tend to trust ourselves, so when we do something (like read an article), the default trust is pushed above zero, just because we're the one who read it! This effect is why most people think they are above average drivers, for example.

So: reading an article about an ad campaign, that contains the ad and its claims, can affect how you feel about the advertised product... even though you know it's an article about an ad campaign!

In marketing this is called "earned media" and it is valuable because it hijacks your trust for the media source to support the ad claim. A spectacular example was the "Swift Boat Truth" ad against John Kerry, which barely ran on late night TV, but was widely covered by the news media. It had a huge impact on Kerry's campaign for president.

I think it was an observation of their mental process at work, in which we tend to remember things we read as facts even if we do not know they are true (or even if we know they are not true at the time of reading). In other words: the change of opinion was not necessarily rational, why is why they said "It's amazing how well this works despite being obvious propaganda."