Not true. He was a socialist but left the Left because of their devotion to Stalin in spite of his atrocities. He loathed the Left for covering up for him. He split with the Left after the Spanish Civil War and went on to write in praise of American democracy, Jefferson, etc. He wrote great novels of outrage against the system, including sticking up for the little guy against the union goons in MidCentury and against the super-government of the New Deal in District of Columbia.
Even the USA trilogy and works before are hardly communist. As we say to New York Times journalists, check your sources.
See DFU's post #34. In the 20's and early 30's while not formally a CPUSA member he was certainly aiding and abetting the cause. During the later 30's and 40's he was still a "man of the left" a la Christopher Hitchens but like Hitchens began to show streaks of Libertarianism. Where he was exactly in 1945 was anybodys guess but he was almost certainly not a conservative.
Basically what he was doing here is a call for the establishment of what eventually became the Marshall Plan.( A Big Government/Statist program if ever I saw one.)