I believe symbolic here means that it won't actually function to deter illegal immigration, at least not proportionally to spending that money on other efforts.

Physical things can be symbolic...

A wall symbolizing keeping out foreigners without actually doing anything useful to do so is pretty much the definition of symbolic.

yes, I meant that the wall will physically exist but it's more of a symbol - a KEEP OUT sign of sorts - than an effective tool for border security... or at least not a cost-effective tool.