I see your point. Maybe the stewards could check inside the flask? If it’s a bomb, don’t let the person in. If it’s tea alles gut. After all, this is what will happen should they find any suspicious items when checking bags and patting down.
I was once told the issue is more to do with bottles with lids being a potential hazard in the event of an evacuation ie you tread on one with a lid you might twist your ankle. If it doesn’t have a lid and you tread on it, it will collapse.
I brought some slices of spinach bread. Upon them being discovered on my person, the security guy paused, and told me I could take them in but could not eat them in the ground.
Whilst Barber’s program notes clearly state his issue with bringing your own food into the 1901 lounges / Dicks Bar, which I understand, some stewards seem to have interpreted this as also being a ban on eating your own chocolate bars, and a slice of cake, in the seats.