To answer the original question - in general the article is a good description of the majority of independent schools. However I dont think the influence of the pupils can be underestimated or understated. Pupils in independent schools are generally very different to those in state schools and the issues of teaching them, and thus the methods ( as well as what makes a good teacher for them) is very different.
Classroom management and discipline are not central to teaching in an independent in my experience. However, lacking good subject skills or good skills in instruction and teaching are important factors. One big factor I found moving from a state school to an independent was that the pupils would eat the work. They were keen, they wanted to learn, there was little disruption
(and I am sorry to say it but being the children of gentlemen and women did not necessarily make the intelligent, many were struggling to be average).
The fact is, the lack of disruption , low level and more hard core makes an enormous difference to what can be taught and learned.
As to the issue of qualified. I think its far more complex that a piece of paper with "QTS" on it. Nor is it an issue of how old and fuddy you think the teachers are (often experience is a key factor in doing well at this job).
In my last state school a whole load of older teachers were replaced on the grounds they didnt sing and dance like the kiddie ones for ofsted and didnt do " fun" lessons but I can tell you the exam results and the discipline took a down turn from the minute they left. It hasnt recovered.
I also think before you ask about " being qualified" - you need to look at what exactly being a wualified teacher really means. In my school we have lost a number of staff (always a worrying sign) in the last year or two ( yes there are reasons and , yes it does mean the school is going downhill I hate to say) . We now have a geography A level taught by a historian ( parents do not know) but she is a qualified teacher! Knows little or nothing about geography....... we have a historian ( another) teaching Philosophy and Ethics but who really wants to teach sociology and psychology ( neither of which she is qualified for in any way) but she is a qualified teacher...... we have a PE teacher teaching maths ( not his subject). I also noticed we have a raft of teachers who are "QTS" and teaching A level who never took A levels themselves. They were late entrants to teaching who took Access courses. They have no O levels ( GCSE) and no A levels and are relatively clueless about teaching at those levels but hey, they are qualified teachers!
Bring back the old days when we had real graduates who taught their own subjects and inspired their pupils. Also bring back pupils who has courtesy and good manners and knew how to behave and even if they could not learn themselves, they could at least allow others to do so - unlike most state schools where the ethos is " I dont want to learn and I am going to make sure you dont learn anything either".
At least that isnt ( yet) part of the independent school scene.