My first impression is that the label, "ergonomic" is at lot more marketing than real. When at rest at a table, my hands are generally flat. That is, the palm is not near vertical.
Second, there is debate about advantages the non-dominant hand/limb (NDL) may have over the dominant limb (DL). I am also a lefty. During my training, it was said that the NDL is better at some fine movements when proprioception is important, such as changing a radio dial without looking at it. Of course, if you are driving an American car, the radio is usually on the right, so experience may be a factor too. I do find it awkward to use my left hand for that. When adjusting an oscilloscope or other instrument, I use my right hand in preference. A notable example of a right handed person using his left is Phil Mickelson. He is a well known American golfer who is right handed, but plays golf with left-handed clubs.
From my very first experience with a mouse, I used my right hand (NDL). It just seemed natural to use my index finger to "point." Isn't that what an index finger is for? It also allows me to take notes, etc. with my left (DL) while controlling the mouse at the same time.