I think DLE editors chose the most probable etymology and discarded the less probable, at least at the moment when the entry was last edited. Maybe there is new evidence and the entry should be updated, but in the 1980s the situation was as follows.
On one side, there was this sort of oatmeal called hormigos, also a dessert with crushed nuts, whose name maybe comes from the resemblance of its small solid pieces with ants. Theory says hormigón takes its name from its similitude to this food.
Also, there is an obscure synonym of both hormigos and hormigón, nuégado, which shows the same relationship.
This is the most plausible theory.
On the other hand, there is the Latin noun forma, meaning 'framework' (the wooden frame inside of which you pour concrete or mud and let it harden, to create a slab). A verb formĭcare, meaning "to build using this kind of framework", could have derived from it. Then, it became Spanish *hormegón, and from there hormigón, by "contamination" of hormiga.
The problem with this theory is that there are no traces of this theoretical verb formĭcare, except only in a French work from the 11th century where it is used as a synonym of formare. There is no other testimony or descendant words in Romance languages.
This is the least plausible theory, for lack of evidence.
But maybe now (2018) there is new evidence for the second theory, although I doubt it.
Source: Corominas y Pascual. Diccionario Etimológico.