I think that is bad advise. Many mechanics subscribe to the theory that putting new oil in an old transmission will cause it to fail. There is some slight truth in that, but the odds of that happening at 105K miles are much lower than the odds of it lasting another 100K miles without a fluid/filter change.
His thought process is that flushing the transmission can break off deposits and cause bigger problems. I read that some people doing the transmission service at higher mileage will do the drain and fill as opposed to the complete flush (I assume with a new filter and gasket as well still?). Is that the safer bet in this scenario?