Yeah, but it still stands for known languages.
But it would be almost impossible to decrypt any document encrypted using the Abu Dhabi cipher," Al Mazroui said.
"Almost" is not a real good word to use around cryptography.
If you have a fixed set of symbols that you use on a block of text, those symbols are still going to be used in whatever frequency is typical for the use frequency for their substitutions for that language. What he needs to do is to have the letter substitution be changed according to some predetermined standard such as the first 100 letters of the message are encoded by the novel alphabet where A to Z = 1 to 26, the second 100 are encoded where A to Z = 5 to 26 and back through 4, etc, varying both the numbers in succeeding groups and the starting point in the novel alphabet for each. If one doesn’t know the varying group length or the start point for each group in the novel alphabet, one can’t easily look for any kind of frequency distribution for particular letters such as “e” or for common letter combinations such as “ed” “ing” “ly” and double letters such as “tt” in “letter,” “bb” in “bubble,” “dd” in “added,” etc.