By which way, your i7-8700 and R7 2700X are similar CPUs? Other than they both being CPUs, they are completely different. With different: release date, core clock, core amount, thread amount, architecture, socket type, lithography, price, and even different manufacturer for god's sake. You can not possibly ask for the same temps, using the same cooler, out of your i7-8700 as other people are getting out of their R7 2700X.
It's like when you have Audi A8 and other person has Porsche 911 Turbo S and you're asking for the same 0-60 time (5.4 sec vs 2.7 sec), while pointing out that both are cars and must have the same performance. Really?
Can you give me some tips on how I could get my temps to the temps that the other people had without another cooler?
Well, since you want the same temps as you read from amazon reviews, ask them on how they managed to do that. I don't know how they did it.
However, the little what you can do, is adding more, high-end case fans to improve the airflow inside your case. You can also flip the fan on the rad around, so it pulls the cold air in from the top. But that works against natural convection of hot air rising and also creates high positive pressure inside your PC case. While positive pressure is better against the dust intake, it is also worse at cooling than negative pressure is.
Also, if you would have big-sized CPU air cooler or 280mm /360mm rad + ambient temp is 20-30C then, yes, you could get your CPU idle temps around 30C. Otherwise: No.
If your ambient temp is e.g 35C then there's almost no way on earth, where you can get 30C for CPU idle temps, regardless which CPU air cooler, AIO or open loop you use. Since all those coolers are cooled by ambient air. Only way to get your CPU temps lower than ambient temp is when you're using LN2 cooling.