Cross section images clearly support Chris on this where you can actually see the depth of the hardening going all the way through and rising quite high on some examples.
Cross section is juste cross section nothing more. It only show an inifinite part of steel, it's the same when you look at water of a river or a lake that you see is only surface but not that there under surface, when you are front of a closed tunnel, you see the door not the tunnel. On the over hand, if you look at the picture 9 or 10 on this link below you will see a little triangle just above the edge, we can reasonably speculate it is the hardened part of the blade; how thick is the upper part of this triangle ? a blade has usually a wideness of around 3cm and a thickness of around 7mm at the mune. I guess it is not more than 3mm. http://legacyswords.com/nihontoCS/crosssections.htm
Steel below a certain carbon content will not form martensite
True, and which percentage please ?
In any case, your original comment that the yakiba is only on the surface is simply incorrect
Top layer was not the best term yes, but further, i precised not more than 1.5 mm depth.