The definition of eclipsing binary star systems
I assume that the diagram indicates what the observer sees (if they had a big enough telescope!). i.e. The viewpoint is nearly in the orbital plane but not quite. Why then are the eclipses asymmetric, with the secondary eclipse being shallower than the primary? Well probably because the surface brightness of the two stars is different - i.e. they have different surface temperatures. The secondary would be cooler, so when its surface is
obscured by the primary, then that results in less flux being eclipsed. Note that this would be the case whether you were looking along a line of sight exactly in the plane of the orbit or at a small angle to it.
There is no definition of an eclipsing binary that says the plane of the orbit must be in the line of sight from Earth. It has to be close to that in order to get eclipses, but it does not have to be exactly true.