Vacuum flasks actually have a legitimate purpose, outside of trying to killing people. The police, supported by the Swedish society, did say they were taking steps to reduce access to guns and explosives after a teenager was killed in gun violence. You only think that extreme because you are using America as a baseline. In America, we watch a gunman use assault weapons to gun down dozens of kindergartners, and we accepted it without any additional restrictions on the access to these weapons. That's our baseline, so everything else appears extreme.
People will use the most effective tools available to do a job. If they don't have access to fully automatic weapons, they will use semi-automatic assault weapons. But it will be less effective. If they don't have access to semi-automatic assault weapons, they will use semi-auto weapons without some of those defining characteristics assault weapon characteristics. But it will be less effective. If they don't have access to semi-auto long guns, they will use other guns. But it will be less effective. If they don't have access to any guns, they will use vacuum canisters with explosives, knives, cars, baseball bats, etc.. But it will be less effective.
No one thinks reducing access to more effective weapons will eliminate murder or make people get along. It will just make their ability to kill others less effective. That's the goal. We are tired of incompetent virgins having the ability to murder dozens of innocent people without much effort. And that trend (the effectiveness of weapons) will get worse as technology improves without any new regulations. There is a reason that bombing is not the preferred means in the US - because it's not very effective. Consequently, the Swedish murder rate is about a fourth of the US rate. So it does make a difference. Better is not perfect, but is still worthwhile.