In other words, doing well in the technical aspects of a student drawing course is not even close to demonstrating that one is "great" on the same level as, say, Caravaggio or Rembrandt.
The things you read on this website sometimes... Accurately copying Bargue requires far more that mediocre skills. Caravaggio is a great painter in the sense that Mozart is a great composer. If that’s where you put the bar, I probably have enough fingers to count the great painters in the whole human history. Amusingly for this discussion Picasso might still be amongst them.
That said, Bargue's process was intended for communicating good taste to 'lesser' commercial and decorative artists as much as it was used for training fine artists. It's a method that essentially boils down to copying pictures of sculptures which are twice removed from nature.
Before and since Bargue, we've had great artists who gained their chops by being students of nature. Many of them never saw a Bargue plate. Many of them didn't use the sight-size method. And there were a lot of skilled Bargue disciples who never had any real impact on art history.
All that to say, I don't think the parent comment is at all out of line. Copying Bargue plates is a fine warm-up or substitute for life drawing, but it's neither necessary nor sufficient to becoming a great realist (we had those long before Bargue). Plus, the world has churned out countless nobodies skilled in the art of copying Bargue plates (which I say as someone who would like to be one of those countless nobodies). It's just not as important as people think.
Regardless, I've done several Bargue drawings, and they're wonderfully meditative once you get into them. You don't need an atelier, though it would help. You can find copies of the plates online, and many videos of artists doing one, to show you the method. If you want further guidance, email me (look in my profile) and I'll set you up. I've actually run lunchtime classes at work making Bargue drawings.
Just as we can't really say he was a great realist, we can't say he was a mediocre one. He didn't make realist art. We can say that he mastered the skills, so if he'd worked as a realist, he'd probably have been pretty great, if only because once he found a preferable mode of expression, he was actually great.
I'm not sure the original poster meant "great" in the sense of Caravaggio or Rembrandt.
To a lot of people being technically proficient is all it takes to be a "great" artist, so by that standard Picasso would qualify.