The fallacy is called "speed":
Some companies are just looking for young, smart people who can solve problems fast.
This fallacy has some correct side: Software development is most of the time about solving problems and when you can solve them faster, you will have the software faster.
The fallacy part is, that fast is the only quality you need.
With many, smart and young developers right of the college, you can create a huge pile of software in short time ... but you can also create a huge pile of problems in a short time. That is the reason, there are so many quality problems and security troubles around. Those young, smart people just lack the experience to make good design decisions ... and those decisions do not end, when the rough design is done.
Those people make fast decisions, but they just lack the experience to oversee the consequences of those decisions.
Without also having experienced developers around, which can share their experiences and guide other people, many companies are doomed to have bad quality software also in the future.