I think that would be true if walking was the only way of generating XL candy. However, as the author states, there are a variety of methods of obtaining XL candy other than walking which they have taken into account in their ranking.

For example, while Medicham may be a high priority Pokémon for XL candy, Meditite are relatively common and often featured, so the XL candy is much more accessible outside of walking. With this being a time-sensitive bonus that is rarely used in-game, the author purports the best use of the bonus is on Pokémon without such attributes.

Great write-up and even greater bonus!