[frankc]

I think it's a little more complicated then that. A variable might not be linear in general but may be approximately linear within a certain range of values. You might fit the model on values only within that linear range and thus get a good fit. The model may be very useful inside the range of fitted values but garbage at extrapolation. As long as you understand the limitations it can still be a useful model.

[jfarmer]

While you’re right, the original post is meant to be pedagogical. Someone who doesn’t understand the fundamentals of model selection might learn the wrong lesson(s).

You kinda have to expect a student to use the examples you give.

There’s really no upside to using that example.

[zwaps]

If I were the author, I would drop things like temperature unless I am willing to discuss nonlinear transformations thereof.

Instead, why not simplify it down to D=Freshness of Coffee Beans ?