Why reinvent the wheel? Unless you've already created your own pattern library to draw from, you're not going to save time writing it all yourself. How are users going to benefit from your hand-rolled code?
There may be plenty of good answers to these questions but they should be asked.
I'm pretty sure using bootstrap is re-inventing the wheel more than hand coding is. Using a framework which when installed and configured gives you the end result of a "wheel", as opposed to just coding up the wheel using HTML/CSS. Let's also make it clear that hand coding doesn't mean sitting there repeatedly typing out every character on every new job. There is always previous jobs and code to draw on to flesh out the needs of a page or site build efficiently and reliably. The benefit for developers is that you have very detailed control and knowledge of the frontend, rather than some broad "does Bootstrap do this thing I want" buffer between you and your own work. Each to their own. No right or wrong.
I think they're saying Bootstrap's wholehearted adoption of flexbox is not an enticement to use Bootstrap, they'll stick with hand-coding designs, using flexbox where appropriate.
Why complicate my life?
I do love flexbox, but I would rather simply code it by hand rather than have some tool/framework/precompiler do it for me.