But since it’s a “Color Output” socket, isn’t it clamped to the interval [0, 1]?
No, but that is what you would expect, so that is why I explicitly mention that.
Simple example, showing this:
And not:

With a “Vector Math” node, the “Separate XYZ” node would really just isolate the Z-component.
Yes, but he puts a value (from the Math Node) into a Vector socket, turning it into a vector with three the same values and then he uses one of them….So, like: a is a number, then make a vector (a, a, a) and then consider the third position of that vector (the Z)..
Personally, I think that is stupid 😉