Mostly a waste of time.
The extra trash seems to show that the tank was Kreemed at one time and possibly why once exposed you see far deeper rust inside like the last few seconds of when done and showing the end result.
A waste of time doing all that shaking with no abrasive inside, the small amount of metal parts put in were a joke, they really did little. Use like coarse type aquarium gravel and 10X the result with shaking maybe 5 minutes max. It's all in how many small sharp edges you put in there, 10 bolts/washers are a waste of time, you make the sharp pieces do the work for you by increasing the number of them. Incidentally an excellent cleaner for plastic car radiator reservoirs as well using gasoline as the solvent, the tanks come out perfectly clean like new.
To remove rust use salt mixed with vinegar but it will eat enough rust you may spring leaks where it was the only thing covering them up.