I guess the author was thinking of how cat's original purpose is to concatenate multiple files, not show just one file. (But I certainly don't think using cat in the latter way is a misuse.)

There's also a commonly noted "unnecessary use of cat" where people do this:

  cat file.txt | grep foo

instead of this:

  <file.txt grep.foo

but that's not relevant to bat (which can be used unnecessarily in the same way).