This reminds me of the writing of C.S. Lewis. Lewis stated that if you look at the world, two things are clear: that there is a certain way people ought to behave, and that people often don't behave that way. Sidney is making a similar observation, in that there is a way that people act throughout history, but it may not reflect how people ought to act. Historians pass no judgement, they merely report.

So it seems here he's basically saying "yeah philosophers are cool and all, but they're basically just saying things that few people can understand, and fewer people can actually apply to their lives." It's funny because I can imagine someone from present day saying it, just less poetically.