Also, fairly reputable sources that produce mostly good quality journalism will slip in misinformation or hit pieces at the behest of political and corporate entities.

Examples are the New York Times writing a hit piece against Tesla, where it turned out they had driven the car in circles in a parking lot for hours to get the battery to run out, but didn't think that Tesla might be able to get data on what they did. Turns out the author of the article works for the oil industry. The NYT also regularly publishes as fact things told to them by the Pentagon and three letter agencies, rather than calling the information alleged.

major news outlets like wapo have a lot of issues, like so many issues, but making up sources is not one of them.

They don't have to make up a source to be taken for a ride. Look at CNBC eating up the story about two guys (Ligma and Johnson) who were supposedly fired. If the source is anonymous we can't check they worked at Twitter.

Generally more than one person is checking employment documents on published news stories (not tweets). Not saying it never happens but it just doesn't happen with enough regularity to assume every anonymous source is fake.