I do not think that anyone should be misled by thoughts of the gliding qualifications of Bonin (FO, PF). All I have seen is that at some time before the accident he obtained a French glider pilot’s licence. He was qualified (at a point in time) but not necessarily “highly” qualified, nor necessarily current.
More to the point, however, there have been enough stall/spin accidents to glider pilots of all levels of experience and currency to show that no class of person is immune to failure to recognise and recover from such departures when they happen unexpectedly, invariably as a result of pilot error/mishandling. I had access to all the UK gliding accident reports for many years, and one included a fatal accident to a 19000-hour ATPL who converted to gliding and spun in when flying solo. Almost every year in the UK there was a fatal accident from stall/spin, not corrected properly by the glider pilot. Sometimes more than one.
A rare survivor of an inadvertent spin at low level told me exactly how it had happened; he was fixed on a wrong diagnosis of what had gone wrong (as I suspect Bonin did) and never reverted to his training. (In this case, he had a cable break, during a winch launch, and with incorrect pilot reaction, the nose dropped as it would. He wrongly thought that the tail had fallen off, forgot all his stall/spin awareness training, and remained convinced that nothing he could do would restore normal flight. He was lucky to survive.)
By the way, ignoring warning sounds and other indications when one has a wrong idea of what is happening is far from unknown. There are sufficient numbers of recorded events demonstrating this. I am not an expert, but I have read that the auditory channel to the brain is the first to get blocked out under overload. It is well known in instructing circles (or was when I was an instructor) that the only way to get through to a student pilot in brain-freeze mode is to physically get their attention. Speaking/shouting becomes ineffective. Another classic example is the video clip of three people in an aircraft, at least two being qualified pilots, who ignore the persistent undercarriage warning noise and land wheels-up.