Greetings, Shawn! Thank you for the question.
Nemeth Code section 77.iii (about non-use of the subscript indicator) does not apply to your situation, because the abbreviation SD (Standard Deviation) is neither a "letter which has a separate identity" nor "a two-letter abbreviation for a chemical compound". The abbreviation SD for Standard Deviation is a two-letter abbreviation for a concept or value. So, you do need to use the subscript indicator before the subscript numeral 2 that applies to SD. Similarly, you do need to use the subscript indicator before the subscript numeral 2 that applies to the abbreviation SS and then the baseline indicator before the horizontal fraction line that follows it.
We could say that any abbreviation other than a two-letter chemical compound or a function name does not satisfy the Nemeth Code criteria for omitting a subscript indicator.
In addition, you do need an English letter indicator before the abbreviation SD, because it is an abbreviation that corresponds to a shortform word and is not followed by a period. You're doing it riiiiiight!
I too look forward to an updated rule book with UEB in it and possibly more examples.