“In any event, it is a midas tznius by yarei shamayim as many people above testified personally witnessing Rabbonim and other yarei shmayim”
How do we know that this is being done for reasons of tznius and not something else? There was a time when ALL people in the USA referred to each other (even married couples) as Mr. or Mrs. So-and-So EVEN WHEN SPEAKING TO EACH OTHER. It was just their way. Many Rabbanim simply refer to their wives as Rebbetzin. That is their prerogative, but it does not make the name an untzniusdig thing.
There was a story of a talmid who went to his rebbie’s house for a Friday night meal and watched with awe as his rebbie ate exactly four spoons of soup from his bowl and no more. The talmid did likewise, because clearly there was some mystical reason for eating only four spoonfuls and no more, and of course, most of his soup was left in the bowl. After dinner, he approached his rebbie and asked him, “Rebbie, I noticed that you ate exactly four spoonsful of soup tonight. Please tell me, what is the significance of doing that? Is it kabbalah, or a segulah?” to which the rebbie replied, “I used a bigger spoon than you did.”
Sometimes there is a very simple reason for things that we observe gedolim doing, that has nothing to do with halacha, but is just a personal thing. I am not saying this is the case here, and there MIGHT be a tznius inyan for them, but maybe our Gedolim feel the need to behave a little more formally than the rest of us.