Sorry for the confusion, Content Builder/Cornerstone's section, row, and column are still using the conventional CSS properties. However, if you need to, you can write a custom class that applies a flexbox properties to your section, row, and column.
You can checkout the guide here https://css-tricks.com/snippets/css/a-guide-to-flexbox/ or https://theme.co/apex/forum/t/using-flexbox/24851
Just to let you know, this has already been added as a feature request for future development. All of these items are discussed with our team internally and prioritized based on the amount of interest a particular feature might receive.