It's normal for articles like this to conflate income and wealth whenever it's convenient to whatever point they're trying to make.
For example, one might see an article complaining that some percentage of wealthy people didn't pay income tax last year. But an income tax is a tax on income, not wealth, and it's commonplace for wealthy people to have a bad year and lose money.
The wording also (deliberately?) suggests that "they" in 1980 is the same as "they" in 2014, which of course is far afield of the truth.
Americans enter and fall out of the 1% all the time. Some significant fraction of 1%'ers are there for the one and only year they sell their home, for example.
"Some readers wondered if the 1 percent by wealth weren’t an entirely different group of people from the 1 percent by income. But there is substantial overlap: the Fed data suggests that about half of the top 1 percent of earners are also among the top 1 percent in the net worth category."
Is this change in distribution over time being measured using income tax returns? In the 1950s the highest marginal rate was about 90%, and there were many loopholes and special rules. How do we know those rich people back then were accurately declaring income above $250,000?
Not true. The highest income 1% have seen their share of national income roughly double. But the highest income 1% is not the richest 1%.