Aiming at C++11 is not a reasonable definition of "modern", as it was C++'s second ISO standard that was published a decade ago and which has since seen between two and three major updates (depending on the opinions on c++14)
It's not the "aiming at C++11", but rather "Write code that does not look odd in a code base that is using constructs from C++11, C++14, C++17, etc." - The library uses C++11 to implement an API that should not feel weird when used together with other containers from the STL.
C++11 was a huge shift in how C++ is written, and term coined for "code written using the new techniques" was "Modern C++". Whether you think that term should instead mean "the latest C++ version" is a different matter altogether.
In C++ land "modern" has become synonymous with post 11. Effectively a domain specific definition. Reasonable considering the difference between pre and post 11. Pre and post 20 will probably be treated similarly in a decade