There are always lies, but maybe "lies" isn't the right word. There are always discrepancies between rhetoric, or belief, or expectations and realities. Beard saw the discrepancy between what he and others assumed to be right and the way things really were and he claimed to see similar discrepancies in the past.
In a way, Beard was a frustrated or disappointed idealist. Reality never matched what he wanted it to be, and eventually he came to relish his own disillusionment and cynicism. That was pretty common in his day -- from Mencken to the muckrakers, there were plenty of cynics.
And of course, the beliefs or ideals or expectations people have change over time. In 1789 it wouldn't have been considered shocking to say that the new Constitution intended to impose limits on democracy. A century later, when democracy had been enshrined as an ideal it was considered heretical to see the Constitution as undemocratic, particularly if, like Beard you cast your argument in a cynical, muckraking way, attributing low self-interest to the Founders.
You could see Howard Zinn as another disillusioned idealist. He came from a left-wing background, but it was his participation in the aerial bombardment campaigns of WWII and the discrepancies between the official story and what actually may have happened that soured him on the American government in a major way.
I don't think Beard is really that surprising. You run into people all over the Internet who don't want to believe that any belief that they disagree with can possibly be sincere and on the level. Once they disagree with somebody or something it always has to be a cynical, materialistic sham for them.
Thanks for the post, sorry to say I’d never heard of Zinn or Beard.
In college some professors identified themselves as Marxists, but never told of possible catalyst influences from people like Beard & Zinn.