TL;DR: The Washington Post's article seems to jump to conclusions not inferred by the PEW study.
The PEW study referenced article starts with this paragraph.
"The amount of time that Americans ages 60 and older spend on their TVs, computers, tablets or other electronic devices has risen almost half an hour per day over the past decade, according to a Pew Research Center analysis of Bureau of Labor Statistics data, even as screen time among younger people has more or less held steady."
This
PEW study doesn't compare reading to TV it compares reading to screen time. I do all my reading from a screen. From ebooks to articles most of my screen time is reading. The article goes on to say.
"This rise in screen time coincides with significant growth in the adoption of digital technology by older Americans. In 2000, 14% of those ages 65 and older were
internet users; now 73% are. And while smartphone ownership was uncommon at all ages around the turn of the 21st century, now about half (53%) of people 65 and older are
smartphone owners."
The Washington Post's leap that this might mean increased risk of
cognitive decline may not be quite accurate since the article they linked to was only referring to TV not computers, tablets or other electronic devices.
I'm not going to comment further on what seems like sloppy reporting.