Not true?
Well here's my source (besides my own experience) - an article written by Bob Flexner, a well known published woodworker whose article mentions Brian Boggs, also a well known published woodworker:
https://www.popularwoodworking.com/f...se-so-quickly/
By the way, I am in no way saying dowels are "better" than a mortise and tenon, or any other kind of joinery for that matter. However I do not agree with sweeping nature of your conclusion regarding one being inferior to the other. While I understand the long grain to long grain surface concept, in reality if the dowel joint is well made and well glued, they are more comparable than your statements would suggest. As a source to further elaborate on what I am saying, here is a link to the tests Matthias Wandel did measuring the strength of the two types of joinery:
https://woodgears.ca/joint_strength/dowel.html
I do think a mortise and tenon joint in most circumstances will be stronger, but not radically so. In my own woodworking I tend to use dowels in applications and places where a mortise and tenon would not be possible or practical. In applications that lend themselves to mortise and tenon joinery, I use a mortise and tenon.
Edwin