I suspect the added cost/environmental cost of adding a slightly more complicated setup than panel <-> fixed resistor, such as an mppt would be a worthy tradeoff.
I understand the point of the blog is to reduce the inputs to simplify, especially in eschewing batteries, but I suspect the loss in efficiency of not tracking the max power point would be significant.
I do like the concept though, in terms of shifting heat from daytime to later in the day, and localizing it into a box, rather than heating your whole house. It feels like the kind of thing you would put in your bedroom to help keep it warm into the evening.
Worst days so far.