One of the sources linked is from the United States Environmental Protection Agency, which is a federal agency, so it should be a credible source, but due to political influence can sometimes be questioned. However, the second source linked is from the BU School of Public Health, which is known to be very credible.
In the BU article, it states that fast fashion is one of the most impactful influences on landfills. As the quality is too low to be used for anything else, and people just keep buying to keep up with the trends, and then once they are done with it, it just goes straight to the landfill.
Although clothing is adding to the problem, it is not the leading cause of landfills in America. The leading cause is food waste. "Food is the single largest component taking up space inside US landfills, making up 22 percent of municipal solid waste." https://www.rts.com/resources/guides/food-waste-america/