That is not totally true, This is a mix of 2 things, using the JVM (which like you said is being tuned and optimized for heavy loads) + using a true asynchronous and reactive programming (and IO) model built on great technologies such as (in this specific case: Kotlin, Eclipse Vert.x and Netty).

As an experiment if you would pick another random set of libraries (imagine a servlet container) achieving the same results would not be so trivial, see for example:

https://www.techempower.com/benchmarks/#section=data-r17&hw=...

And observe that Eclipse Vert.x is on the top for these reasons while other JVM frameworks are far behind.