I imagine they mean not in a retirement account that has restrictions (penalties for drawing money out early) but also has tax benefits

Risk/rewards. Banks implies low risk investments that also have low reward. You need some of this as emergency backup - if a big bill comes in. (Big bill could be unexpected like a job loss where you have to buy food from savings, or could expected like where you are putting a little extra away for a vacation)

Investment/retirement implies long time, you can afford to take a large risk because the reward is worth it long term. Stocks historically average 10% any 20 year time, but in that 20 years there are some years where they drop 20%. It works out in the long run, but you better not have near term savings in there because you can't know for sure that your money will be there.