Or perhaps, just perhaps they found it amusing, much like modern-day adults are building sandcastles the size of a bull, or are placing rocks upon rocks, or stack wood, or something.

But that is exactly what they mean by symbolic or ritual behavior. Ritual is, of course, archeology slang for "idfk why they did this". Although it is often defined as something strictly spiritual, it is not always used that way in practice, and can be thought of as more of any physical act, that serves no practical purpose other than perhaps in a cultural or communal sense.

Symbol behavior, on the other hand, encompass all of the things you mention here: sand castles are a prime example of symbol behavior, as is arguably rock and wood stacking, although sand castles are very obviously using one thing to represent another, and rock and wood stacking are more abstract.

Either way, you are saying the same thing they are but in different words.