I've heard the same - "never dethatch St Aug, as it will destroy it". I am not a professional, but I detached my Back Yard St Aug last year, and it was a VERY aggressive dethatch. I had so much thatch and dead/brown grass under the canopy, I thought the worst that could happen is I'd have to replace it... and I have to confess that a little part of me wanted to see it damaged enough to replace it all with Bermuda sod. However as the pics I took around July timeframe showed, it recovered very nicely.
BTW, your results could vary - so if you decide to dethatch based on my results - beware.... they say you can destroy a SA lawn by detaching it... :lol: