My experience is radically different than yours. In fact, it was my dealer who showed me the problem in the first place. Then, he demonstrated it. Then, I experienced it for myself. It cost him a sale -- and he knew it would cost him the sale. (I admired him for bringing it to my attention. I would not have discovered it in a normal test ride.)

HOWEVER, I only did a test ride after a few minutes of orientation from the dealer. Let me say that, although I'm not entirely convinced that this is a myth (after all, I've experienced it), I am willing to doubt my dealer's knowledge of the shifter. PERHAPS he shifted it wrong and taught me to shift it wrong. Both of us got an upshift when we intended a downshift. Judging by your post, we needed to push the lever BEYOND the normal downshift point in order to safely non-shift.

But . . . I guess that leads to another question: Why should you need instructions on a shifter?

And it seems that an inadvertent upshift is still very possible -- just not entirely certain.