While a lot of factors go into determining whether a language is readable I have always felt the most obvious is familiarity. The human mind is very good at adaptation, and often it’s astonishing what we will perceive as "normal." Familiarity only comes from constant exposure, though, which means that languages with relatively simple syntax become familiar more quickly. Lisp is at one extreme, with only one syntactic construct. It’s very easy to become familiar with Lisp, although grasping the large Common Lisp standard library is another matter. I tend to agree with the author that C++ is a language at the other extreme. Most C++ coders I have encountered use only a relatively small subset of the C++ language. Worse yet, everyone uses a slightly different subset.
Of course, the biggest impact on readability comes not from the language, but from the developer. A poor developer can write illegible code in any language. A good developer? I’ve even seen well-written, readable Visual Basic code (once).