Squishy( adjective): 1 being soft, yielding, damp
2 not firm, steady, or fixed ( Definition comes from merriam-webster)
From: “Some people might not be charmed by an experiment that relies on squishy concepts like beauty and happiness.” In this sentence, squishy should mean not steady because it says that people might not be attracted by this results of an experiment which means the result is not steady. People might have different ideas.
In addition, I’ve found another meaning of squishy, to describe a really cute old people, typically little old man, or someone of a huggable nature. From Urban Dictionary. Obviously, those are not best explanations in this condition.
Squishy here is a great example of a word that has different literal and figurative meanings. Although we might say that after it rains, the ground feels squishy–which is the literal meaning in your definition–we could also say that something that has to do with emotions is squishy. It’s as though they’re made out of that soggy ground after the rain–they are not solid, stable ideas but instead ideas that change from one person to the next, or from one day to the next. This would be the figurative meaning. In both examples here, think of concrete as the opposite of squishy.
Do you see how the Urban Dictionary definition is an extension of that figurative meaning?