Nothing to be emberassed about as these things aren't discussed that often or aren't documented in an other way.
"Compute terrain --> Displacement shader to vector --> Length to Scalar" should give you the scalar value of every point's vector.
Scalar = point in space with a value/magnitude
Vector = point in space with a value/magnitude + direction
So the reason that this works is because for an altitude offset function you need "altitude" which is a scalar (hence, altitude has no "direction").
What you need is to have the terrain represented in scalar values for displacement, so you need to strip the vector of its direction as this is not important and not applicable to a scalar.