Thank you so much for the kind words!
Thank you also for how insightful your comment has been.
SPOILERS BELOW
The solution that i had intended to design was in fact the solution that you found. Sadly, this solution was not obvious, for the reason that you have mentioned. I suppose there were really two factors as to why it was left in the condition it was. The first was predominantly the time constraint, as that level was infact made a few hours before the jams eventual conclusion. The second was that i postponed changing the level due to conern that I would deliberately force the player into an unclear and unexplained solution. I also feared that the game's difficulty would become too great and my inability to create a smooth transition into what is, a trickier puzzle would only bring annoyance. As a solo dev, I had no playtesters, which left me second guessing the correct difficulty to leave the game in. For that reason, I left the level in a potentially incomplete state, whilst trying to think of a way to show the player they could trap boxes in the way you described. In short, I completely agree with all the points you made, and hope this has shed a little more clarity into the fact that leaving the level in that state was not a deliberate choice.
Your comment is the first to point this out, so congratulations for finding the rather hidden solution, and finishing the game. It means a lot to see someone finish your game, and have such kind words to say about it.
Finally, i want to thank you for sharing your philosophy. I will look into Patrick's Parabox. I am rather new to puzzle design, having mainly been a puzzle enjoyer rather than a designer, so these resources / case studies will greatly help.