With only a SINGLE intake and SINGLE exhaust? It's little wonder.
If you get that case, you will ABSOLUTELY want to not only put two intake fans plus two exhaust fans, but you'll also want them to all be replacement fans. The stock fans are not premium really. They are cheap, as you'd expect for any fan that gets included WITH any case. I mean, they are better than what is included with most cases out there, but they are not great for a build like this.
I'd put one of these in the rear exhaust location:
https://noctua.at/en/nf-f12-pwm-chromax-black-swap
And one of these in each of the three remaining recommended, top-rear and both front, locations. Another benefit of the larger, more premium S2, R6 and R7 cases is that you can put three 140mm fans in front, rather than only two like on the Define C.
https://noctua.at/en/nf-a14-pwm-chromax-black-swap
People buy cases and expect what they come with to be sufficient for cooling. It isn't, and never will be. You aren't going to get a premium cooling solution for free unless (And sometimes not even then) you pay 200+ dollars for a case. Margins are already low on hardware, they aren't going to just give parts away for free so if you want to build a high end system, you need to be willing to pay for (And smart enough to understand that you need, unlike that guy whose build you linked to) the cooling to go with it.
And if we're being honest here, the 3960x might very much be a configuration that is better suited to liquid cooling. The U14S TR is a very capable cooler, I actually have and am using the non-TR version, with dual fans on it, but it's not THAT capable. It's not going to match the performance of a top shelf (And not ALL liquid coolers ARE) 280 or 360mm cooler. For that CPU, a GOOD 360 or 420mm cooler might not be the absolute worst idea ever.
I mean, that's a 280w part. It's TWICE the TDP of most i7, i9 or Ryzen 9 CPUs.