The argument boils down to "more code, more bugs" so large frameworks are less trustworthy than small libraries.

Of course on the other end these popular large frameworks often have a lot more maintainers (sometimes even more maintainers per line of code) than the smaller libraries frequently only maintained by one or two people. And there are of course synergies (yes I'm using that word unironically) when all the code lives in a single ecosystem rather than being based on a unique combination of purpose-built libraries.

Having used both Django and Flask, and various front-end frameworks and libraries, I can see the appeal of both sides of the argument. In the end I haven't found either argument to be bullet-proof though. Everything is always a trade-off.

This is largely true for someone who already knows the framework(s), probably less so when you're learning. There's less to do to make "a thing" that works in Flask than in Django. That said, many of the times I've reached for Flask I've realized, eh, it's going to be just as quick and "light" to do this in Django. At the working, production level, I wholly agree with you.