Partly...
But also that the Dem party today is significantly more "conservative" economically than we used to be, as the article points out:
In 1910, Teddy Roosevelt thundered his warning that “a small class of enormously wealthy and economically powerful men, whose chief object is to hold and increase their power” could destroy US democracy. Roosevelt’s answer was to tax wealth. The estate tax was eventually enacted in 1916, and the capital gains tax in 1922.
In the 1912 presidential campaign, Woodrow Wilson promised “a crusade against powers that have governed us … that have limited our development … that have determined our lives … that have set us in a straitjacket to do as they please”. The struggle to break up the giant trusts would be, in Wilson’s words, a “second struggle for emancipation”.
Wilson signed into law the Clayton Antitrust Act, which strengthened antitrust laws and protected unions. He also established the Federal Trade Commission to root out “unfair acts and practices in commerce”, and created the first permanent national income tax.
Years later, Teddy Roosevelt’s fifth-cousin, Franklin D Roosevelt, attacked corporate and financial power by giving workers the right to unionize, the 40-hour workweek, unemployment insurance, and social security. FDR instituted a high marginal income tax on the wealthy – those making more than $5m a year were taxed up to 75% – and he regulated finance.
Plus, Teddy was the first presidential platform that used universal healthcare....
So part of it is that Republicans lie and propaganda
But if the modern Dem party didn't think the Dem party platform from a fucking century ago wasnt "too extreme" the modern Dem party would be as popular as it was with FDR.