A harmful modification that doesn't cause sterility will propagate across generations if the indivuals are able to reproduce.
For example a scenario where we see the symptoms but we don't know that they are caused by a gene modification. And where individuals are still able to live and have children.
And I'm not saying that the tech is bad. I'm sure that people can come up with a lot of good use cases.
If it's harmful, it will get selected against, and will disappear rather rapidly.
There's a reason why they need to release 750 million genetically modified mosquitos, and not just 1.
With 6 billion people in the world today, even if our nefarious post-doc somehow gene-spliced 100 people with a 100% probability transmit of a "bad" recessive gene, it would take so many generations for any effect to be felt.
By then, we can assume, it would be trivial to reverse the "damage" caused. Hence the eye-rolling at the idea that one post-doc could be any serious threat.