"Low-tech" is not the same as "unskilled" or "low quality". I have seen low-tech metalwork of superb quality. Low-tech just means that the smith doesn't have power tools (e.g., power hammers), temperature controlled ovens for tempering, access to steel of known composition, quantitatively accurate hardness testers, etc.
Yes, differential hardening with a clay coating is more difficult than uniform quenching. I don't think this blade would have been done that way - probably edge-quenched instead.]
The difficulty in low-tech heat treatment isn't the quench. As you say, you can tell temperature of very hot steel by colour. The tricky part is tempering to reduce the brittleness after quenching. This will be even more the case if the blade is wootz (or some other very high carbon crucible steel); tricky enough so that wootz blades were often air-cooled rather than quenched.
Differential hardening, whether by differential quenching, differential tempering, or laminated/composite constructions like welded-edge/inserted-edge or sanmei add a lot of forgiveness to the heat-treatment process, especially tempering.
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