How do S and L cone cells sense colors like deep blue and red beyond their peak?
The answer is actually present in your chart: color perception depends on the ratios of activations of different photoreceptors. For example, as you go "more red" from 575, the responses of the Green photoreceptors are falling off more quickly than the Red ones. The same is true for any colors in between, and on the other end, where 400nm light would activate only the blue receptors, whereas at 445nm there would be some mix of activation of the green.

From the context of Fisher information it is actually interesting that photoreceptors don't actually provide much color discriminability at their peaks.

For example, around 575 nm, the "red" photoreceptors actually don't provide very good color discrimination: the response to 565 or 585 is going to be almost identical, for example. The way you can perceive a difference, though, is that the response from the green photoreceptors will be very different at those two wavelengths (just consult your chart!). In vision, as in all the other senses, the information is really held in the slopes!