Right, but they hedge their way out of it with "This makes the left and right parenthesis unchangable from within the language (not conceptually, but under some implementations it is not possible)" so it's not even clear which "Lisp" they're talking about. If I'm not mistaken, ( itself is a reader macro in CL.

It's also not that relevant. You'll can add different syntax variants, the Lisp evaluator doesn't see parentheses in any way, just actual interned data.