He mentions "...an adjacent compulsory bikepath, denoted in Germany by a round blue sign." UK is one of the very few (maybe the only?) country in Europe where that round bue sign means "this must only be used by bikes" rather than "bikes must use this". Then he says that where the German cyclepaths are substandard "...there's just a black & white sign, indicating that cycling is merely permitted on the sidepath." So it's a path for walking on which cyclists are allowed to use too, which in the UK is indicated by the round blue "bike and person" sign.
So the UK round blue sign has the same meaning as the German square black and white sign. In most countries the round blue sign would mean "cyclists and pedestrians have to use this" but it doesn't here. It might be more logical if we used a square blue sign with a person and a bike to mean "cyclists and pedestrians can use this but they don't have to, and no one else is allowed to"; along the lines of a square blue bus lane sign, which buses (and cycles and taxis and sometimes coaches and sometimes motorbikes) don't have to use but no one else is allowed to. But we don't.
As for drivers being confused by round blue cycle lane signs, maybe, but I don't think being confused is why they hassle you; they hassle you just cos they're driving. Also, round blue signs don't always mean "compulsory"; for instance, there are some left/right turn signs with plates underneath to say everyone has to turn except buses or taxis or even cycles.