Not long ago there was a discussion about a stall spin mishap where the trainee was a ~40 year old former NFL player and the instructor was a ~60 ordinary guy.
You can shout, you can beat on them, but if they’re bigger enough than you are and they freeze hard, you may well die trying to get their attention.
As well, things can go from OK-ish to unrecoverable in just a few seconds. Knowing when to intervene and not is hard to learn and nobody gets it perfect every time.
if indeed this is some variant of low altitude stall/spin, then once the stall occurs they are already 99.99% doomed absent some really Yeagertastic flying and good luck to boot. So you’ve got to intervene early enough beforehand to prevent the stall. Which means deciding early enough beforehand to have time to intervene. Which means noticing the developing problem early enough beforehand to have time to decide to intervene.
This being a Cessna 208, not an initial trainer, and the company being an air tour company, not a flight school, I expect the “trainee” was a licensed commercial pilot who was receiving the training to become qualified on the 208, and become qualified to perform whatever tour-specific duties the job entails.
When I flew air tours IIRC the checkout was something like 3 flights with an instructor doing normal and emergency ops, then a checkride with a different instructor being evaluated doing the same stuff, then ride along with an instructor flying one tour, then fly one yourself with an instructor riding, then you’re signed off to operate alone.
The 208 comes box stock with dual controls. But if they routinely fly tours single pilot, that means the put a passenger in the co-pilot’s seat. It’s certainly possible they remove the yoke and pedals from that position for that reason. To get them out of the customer’s way, and also reduce the odds of the customer grabbing them at a bad time.
I have a vague recollection that dual controls are not in fact required for even initial training. Ref the old Bonanza with their single throw-over yoke. It’s a darn good idea, but the only certification requirement IIRC is that the instructor have access to controls something like “in a reasonably timely manner”. The word “reasonably” is doing all the work in that sentence.