Yes, it is universal. Of course, a single cable if is correct cannot cause an explosion. Your seller obviously joking with you.
the only thing you need to pay attention to is the quality of the cable. Low quality can bring some problems on load.
Pinouts on different PSUs are not same, so seller might have a point. Even within one brads pinouts might differ from model to model.
I will get from him a 6pin to 2x8pin connector, I'll use it to power up the 1660 and 1660s (max 180 watts for both gpus) hopefully if it works safely, if it dose so I'll mine with it until I can afford a better fully modular psu possibly evga, corsair 1000