Disclaimer-Just a dumb mechanic here who works on semis & has never looked inside a b-box other than the video posted and recommends having a professional make your final evaluation.
It looks like to me if the split washers came out, for whatever reason, you would get exactly what the original poster has described. From what I see the propeller shaft is driven by what I believe the youtuber described as a dog gear, the dog gear is driven by the large gear that slides on the propeller shaft. So if the split washers fail the dog gear & large gear can decouple allowing free spin, and also allow the propeller shaft to be pulled out of the front of the gear housing. I also think this failure could likely show little to no other damage other than dislodged or broken split washers, also mentioned by the original poster. A close inspection of the parts should reveal the cause of failure though. Now, without going into any details, that b-box looks like a terrible design to me and I can’t believe they sold it for aircraft use.