Did you see the experiment? I didn't either. I base what I say about solenoids from many years of both using them on machines back when working and from every book I have about magnetics.He did perform an experiment and compared no-magnetic-circuit with the normal one, and found a difference? Do you simply not believe the results of his experiment?
Like I said earlier there are so many working manufactured solenoids in industry that have NO steel frame around them that making statements about how they are necessary for a solenoid to work is just absurd. The frame around a solenoid is only one way of mounting them, has nothing what so ever with the strength or working of them.
If doing this was actually important to you, you would spend some time reading about solenoids and not watching stuff about them on Youtube. Sorry if that sounds harsh or mean, it's not meant to be but it is also the truth.