Interesting article, but there can be no doubt (with the Venona documents released by NSA and the Mitrokhin documents smuggled out by the SIS) that Hiss was a Soviet agent. The article seems to have trouble acknowledging this.
On Stone, the issue is more complex. The Soviets wanted to recruit Stone, saw him as an "agent of influence" and may have been able to shape some of his writings. He was never recruited, however, and his views, although often sympathetic, were always independent. This does not mean that he was not witting or at times complicit in Soviet efforts to shape American opinion in ways helpful to the goals of the USSR.
The concluding note about energetic young radicals inspired by the Popular Front building the New Deal may distort the environment of the New Deal and overstate--it implies some positive energy from Stalin at a time he was purging millions, and it sounds too much like an effort to justify today's radical agenda.