No, that's not true. The value of scientific knowledge, for example, is in its power to allow us to make reliable predictions about our environment and the consequences of our actions. That's not subjective.
> The historical context is definitely part of what makes art interesting.
Sure, but the historical interest in a work of art should follow as a consequence of the (subjective) quality of the art, not the other way around. If I were to make a painting, no one would (or should) care, but not because my painting would not have an interesting historical context. It's because I have no artistic talent and any painting I would make would be a bad painting. And 500 years from now it would still be a bad painting.
To use a more realistic example: Adolf Hitler and Winston Churchill were both painters, but no one remembers them for their art because it wasn't very good despite having an interesting historical context. Quality art creates an interesting historical context, no the other way around. Or at least that's how it ought to work IMHO.