That definition (apparently) found by searching Google says more about inadequate/bad information found by doing a simple search than it does anything else, I think....
I find it kind of hard to believe that the confusing of the two are still a matter of discussion in "research" because I think there's a fairly common and solid understanding out there about the definition of "introvert".
I've researched the subject of "shyness" online and found that if a person goes far into material (past the stuff for people not generally inclined to delve too deeply) one will find that is a reasonably solid undertanding of shyness in that "more advanced" material.
The reason I've been able to recognize all the "baloney" and inadequate information even in some reasonably "solid enough looking" information online is that I was in some ways shy as a kid, overcame some of it, couldn't overcome some related to, say, public speaking; and yet (contrary to popular belief AND some of what's presented as "research" online) am a very confident person who generally likes pretty much everyone I meet. Or at least I'm confident about what I am as a person. There are things I should not be confident about and rightfully so, so it isn't a matter of not knowing what I should be confident about.
Just from my own experience and off-hand, I think the roots of "reticence" is a matter of young children being emotionally/socially mature very early but parents' not recognizing the need to help a child overcome it at the time in his individual development when that's required.
I have three kids - two sons and a daughter, who is the youngest. My eldest son was called by a kindergarten teacher "an introvert" because he wouldn't "join a group uninvited". She said he enjoyed playing with others, but the "uninvited thing" was the issue.
There wasn't much for three-year-old boys at the time, but I made sure I got my daughter into dancing school at three(where she would overcome the natural tendency to be shy in public situations.
The confidence/sureness I've had since I was about three means a) I wouldn't "perform" as a little kid if it was "just to please someone else", and b) that I'm a sure/authentic enough person that I can't graciously slap on a big smile or do a good handshake because those are what "is expected".
My mother waited until I was school age for me to try to dancing lessons for little girls (three years too late to be right for me).