Historians pull their information from sources of all kinds. For example, a piece of propaganda from WW2 (which is likely to be almost completely untrue) could be an invaluable insight into determining a collective mindset of the time. Similarly, a diary of someone on the street, who's been taken in by that propaganda is also useful.

Adam Curtis exists in the outskirts of a behemoth of a broadcaster, so his interpretation of these events will be useful - especially given that they are broadcast so widely.