I think the point was that seltzer (and anything carbonated, really) would typically be added after shaking. If you add it before, you’re immediately losing all the carbonation when you shake it (and you’re likely going to have your shaker pop open mid-shake from the pressure.) Try adding it after shaking instead, and you’ll find it to be much more bubbly.

And as for cutting down the harsh edge, that’s mostly coming from the crushed ice in your shaker; When you shake, the ice melts and waters your drink down. There are three big ways to cut harshness from alcohol: Sugar, citrus, or water. Adding any of the three will help counteract the harshness from alcohol. So when you’re shaking, you’re inadvertently adding water. Then the seltzer water is just watering it down even more.

And a jigger in a martini glass is definitely not a very small amount. A .75oz jigger is almost a quarter of the 4oz martini glass.