There's no need for a dictionary definition. Just as you just said, diminutives don't normally make it into the dictionary. So, "suelecillo" is a word for "suelo pequeño" whether or not it's included in the dictionary, whether or not you use it regularly. But the reverse is also true. "Bolsillo" is a "small bag", whether or not it has developed a specific meaning that grants it a separate definition in the dictionary.

And whether or not you use it regularly with the original meaning. The frequency of usage does not invalidate this truth: the rules of diminutive formation entail that "bolsillo" is "bolso pequeño". There's no arguing this statement. Just as "mosquito" is "mosco pequeño", i.e. "small fly".

The same can be said for plenty of other words. Words that have developed a specific separate meaning but are still diminutives for the non-affixed original word. Meet polysemy.

Sorry, but "cochinilla" is still "cochina pequeña", "cabezón" is still "cabeza grande", "cabecilla" is still "cabeza pequeña", and "frailecillo" is still "pequeño fraile".

It would be mistaken and a bit obtuse to nitpick on an article which states that "cabecilla" is Spanish for "small head" by contending that no, it means "leader".