Two wrongs don't make a right. Just because the OP might have been ill-advised to backtrack does not then legitimise someone else entering the runway unannounced for a sneaky take-off when noone's looking.
If you are at an airfield where there are frequent PPL training flights then it is naive to always expect the standards you would get from experienced pilots. Students are learning and will occasionally make mistakes. We all had to start at the beginning and the vast majority of us weren't particularly brilliant when we started off!
When I flew solo to another airfield as part of my PPL I was taxiing to the hold when I realised I had made a silly error. This threw me and then I became conscious of another bigger aircraft waiting behind me and I panicked - took off without completing a whole section of my checks. I learned a lot from that - namely do not rush and get it wrong.
But what if this had been a solo student? Mr Pitts wasn't to know there was an instructor on board. Already feeling pressurised because of the busy circuit, the student enters the runway, backtracks (rightly or wrongly), turns, applies full power without looking ahead properly (he is trying to save time remember) and then suddenly finds himself taking off alongside another aircraft who entered the runway without telling anyone.
Piss poor airmanship and quite worrying that others regard it as acceptable behaviour (until of course someone does it to them!)