Hi I'm Clark. I'm the guy in the video.
You are all right to be concerned with the charging and reconnect issues. I agree that a dc-dc charger is not the right solution for this problem for the reasons listed above. The lead would prefer a longer time cooking under the solar panels. I made another video describing the other side of this (from my point of view)
And I will be making one video about something that you guys might find interesting.
I've developed a circuit that disconnects the lithium batteries when they are charged fully and only reconnects when both banks meet at the same voltage so there is no problem with inrush current.
This circuit diagram will be made available for anyone who is handy with a soldering iron and I might do a production run for resale if there is interest.