Okay, I'll take your word. For what it's worth, guilt or innocence of the charges isn't what matters to leftists. If you doubt that, look the Hillary Clinton and her e-mails. What matters to lefties is if you are a true believer and if you walk the walk. If you do that they will make every possible excuse for you regardless of the evidence or, indeed, your own admissions. To them it's better for one guilty lefty to walk free than the cause take a setback. Indeed, lefties getting away with crimes is good because it encourages other lefties to commit crimes in the name of the cause with the assurance there will be no consequences. Consider the russians starving because of stupid dictatorial policies in the 1920s - 1930s but the NY Times pretending it wasn't happening because it would hurt the perception of communism as this great thing to strive for.
In the case of Philby, I didn't think lefties thought he was innocent per se, but rather that his sins weren't so bad as to warrant the punishment (well, the one he'd have gotten if he didn't defect). Because, you know, spying for the commies isn't a bad thing if you are a lefty. But if that was mistaken, as i said I'm not a brit so I'll accept that.
It was indeed mistaken. The mainstream left in Britain, as represented by the Labour Party, loathed communism and the Soviet system.
Philby did not merely steal and pass on secrets. Some of the info he passed to his Soviet handlers caused the deaths of many agents and freedom fighters. Philby truly had the blood of many persons on his hands. But because people found him so charming, and because he was "the right sort of people"; everyone refused to believe the truth about him.
In the film clip that is linked in the original article, they show a bit of footage of Philby in his mother's house after he had been cleared of being "the third man" in the Burgess/McClean spy ring. He was guilty, of course, but his friends in MI6 refused to believe it. For one thing, if Philby was a commie spy, it would mean that he'd been milking his friends for years for information that he supplied to the Soviets (which is what Philby did). His undoing was via the route that is usually a double agent's undoing: a Russian defector gave info of an inside man in MI6 which pointed to Philby.
When he defected to Russia, his promised colonelcy in the KGB was not waiting for him. He hated the cold of Russia and missed the country he'd betrayed. He could not understand why so many people lived in poverty in Russia, the land with the perfect system of government. I suppose part of his folk hero status is based on the fact that it is impressive that he got away with treason for so long, and that he was so charming and likeable. In the clip in which he addresses the East German agents, he still comes of as charismatic and likable. Philby's life as a double agent is a strange story to this day.