I am not the source of the original statement, but my own interpretation of "powerful" aligns with simplicity and flexibility. Plain text has only slightly more structure than a stream of bytes, meaning it retains a lot of simplicity and flexibility. Yes plain text can be inefficient and is overused (a proprietary unpublished wire-format does not need to use JSON). However, 50 years from now, the only data I feel comfortable knowing I'll be able to read is plain text (and possibly also JPEG and a few other well-specified and simple binary formats).
Many binary formats are effectively defined by large complex and transient code bases that target particular tool chains and APIs. A new binary format needs to justify itself, less so plain text.
