If these teachings alone were enough to liberate everyone it's all the Buddha would teach...but it's clear that the Buddha didn't teach just this. The Buddha tailored his teachings to his audience and the circumstance. I don't think these teachings would have worked for everyone. Others might have been too caught up with the idea of a "self" that teaching them this would make them shake there heads and not listen to the Buddha.
Although, I do think these teachings touch at the core of what the Buddha taught. That through mindfulness we can see the true nature of things and through that one sees seeing as just seeing...instead of all the stuff we layer on top of it through our greed, desires, etc. When I look at something attractive desire arises, a sense of suffering arises too if I can't get the thing I'm attracted too...then a whole causal chain of emotions, feels, etc. It could lead one into a frenzy if they lack control. To be disciplined enough to really see seeing as just seeing, hearing as just hearing and so forth seems to require, at least from what I can tell, a pretty disciplined mind that's well trained in concentration and mindfulness. In that sense, the teaching is challenging, because that training isn't easy.
So yes, I do think this teaching touches at the core, because it touches on dependant origination and that's truly one of the core concepts in the Buddha's teachings. It's through the senses and the causal chain that we're bound to suffering and it's only through relinquishment that we free ourselves. That's what the Buddha is getting at when he says see seeing as seeing, hearing as hearing, etc.