Monbiot knows that all the planet-threatening issues are linked to the hegemonic, US-led capitalist/neoliberal/imperial system and the concentration of wealth/power in the hands of a small class of corporate oligarchs. He also knows that only mass, grassroots movements can challenge that system and that Corbyn represent one of the few such challenges in western mainstream politics. And yet despite knowing all this, he was happy to lend his weight to the state's psyop-smear campaign against him, just as he has been willing to smear other radical critics of that system. So he knows what the system is, but is willing to undermine ideological and political challenges to it for careerist reasons. Which tells you all you need to know about his integrity.
As to the Guardian's apparent willingness to challenge capitalism in general, their smear campaign against the one politician who challenges neoliberalism and the one journalist who most threatens the secrecy of the establishment's crimes tells you all you need to know about that. To say nothing about how they have purged most of their dissidents and become largely a mouthpiece for the oligarchy on critical matters of economics and imperial war. It's just a little progressive posturing in deference to shifting public attitudes. At least that's how it seems to me.