What does the “nose-high” product of inertia represent?
The $I_{xz}$ moment of inertia is a measure for how the weight is distributed in the xz plane.
You can obtain it by multiplying a unit of mass $dm$ by its distance in $x$ and $z$ from the center of gravity and taking the negative of it.
For discrete masses (blocks of mass) it looks like this:
$$ I_{xz} = \{-(x_1 \cdot z_1) \cdot m_1\} + \{-(x_2 \cdot z_2) \cdot m_2 \} + \{-(x_3 \cdot z_3) \cdot m_3 \} ... $$
If you look at the plane in question, you can see that it has a nose - which we can consider as one of the blocks - which is

Far in front of the c.o.g. (large value of $x$)
High up from the c.o.g. (large value of $z$)

If we plug this in the formula, we see it leads to large values of $I_{xz}$.
Similarly, there is also a lot of mass in the lower-right corner

Far behind the c.o.g. (large  negative value of $x$)
Low below the c.o.g. (large negative value of $z$)

In essence, everything that is on the x-z line contributes to this value, and as we see from the marked area in the image of the Valkyrie, there is a lot of mass around this line.
If your aircraft is symmetrical around the x or z axis (in terms of as much mass above/below front/aft the axis) you will get a low value of $I_{xz}$.
I guess that is the case for many aircraft, so they make the assumption of $I_{xz}=0$.
More information about the rotational moment of inertia can be found here.