Keyboard shortcuts aren't as convenient with the mouse on the left. The most useful ones are all biased to the left of the keyboard. Left Ctrl is also easier to hit reliably.

In school for fun when it was boring I many times would do writing exercises with the right hand (like write a line of A's, a line of B's, etc) and after a couple of weeks I got the right hand up to maybe 80% speed and accuracy of the left, but I realized I needed to do constant training to keep it as good as the left (admittedly the left had daily training of many hours due to having to take notes, etc). But it does sound plausible!

I would have been left handed when an early childhood injury caused me to switch to right. A few years ago I thought about that and tried re-learning some skills and tasks with the left hand.

It's specific to each task but I can normally get the left handed version as good or better than the right. I am willing to bet most people could do this, you just have to spend a bit of time awkwardly re-learning.