It was one of the image results from the images.google.com search "biomes+latitude vs altitude"
https://www.google.com/search?site=&....0.WcQuprYsjtY and shows the classic altitude vs latitude relationship.

I do agree that it's impossible to get a good result without some significant form of climate modeling. A basic climate model needs to take into account axis of rotation and direction of rotation to get the amount and direction of spin on ocean currents and number of poleward cells in the atmospheric Hadley circulation. Then oceanic currents will move the bulk of heat poleward from the equatorial regions (assuming reasonable axis tilt) and exchange heat with the atmosphere. The atmosphere will carry the water evaporated from warmer ocean waters inland on winds (the clouds formed will, of course, reflect sunlight, cooling the planet). Continental size will also determine climatology because water will last only so long in a parcel of air. Huge continents tend to be very dry in the middle. And there are lots of other little considerations in there as well, such as the hard-to-model narrow coastal currents like the Gulf Stream that don't appear at all on coarse simulations. It's a very dynamic set of processes and I've studied them a fair amount over the years. I haven't found one that works well enough for the computational cost yet.

While it is true that average temperature can be very approximately modeled by cos(lat)+b-k*h, it's an insufficient model to work with the processes above because the amount of seasonal heating on land compared to sea is critical. Land heats more strongly than sea, which accounts for things like summer monsoons that pull clouds far inland of where they might otherwise go. Without modeling the variations due to axial tilt (and to orbital parameters and solar variability to some extent), the planet doesn't have seasons. It's the seasonable variability of rainfall vs temperature that's critical to biome models like the Koppen climate classification, if I recall correctly.

FT opted for the simplest thing that could give a user a place to start from to edit in the climate that they wanted. Too many people seem to believe that it's correct, though. Like a lot of FT, it was intended to provide some inspiration, not be physically correct.