Which part of the results are you referring to? It's well-known that reference counting has significantly lower throughput that tracing garbage collectors, so the fact that C++ is outperformed here isn't surprising at all.
So, there is no universal answer to how C++ reference counting compares to GC.
There is a well known answer, that you’re probably referring to, if you reference count every object and you do it soundly by conservatively having every pointer be a smart pointer. But it just isn’t how everyone who does reference counting in C++ does it, and I was surprised because I’m used to folks going in the other direction: being unsound as fuck but hella fast.
There are but critically there are also a lot of ways to not do ref counting at all. C++ isn't a refcounted language, it's a language where you can use refcounting (shared_ptr), but you don't have to (unique_ptr, value types). It's not even recommended to be primarily refcounted.
They chose a really odd subset of C++ to use here (shared_ptr exclusively), very unorthodox and not something I've ever seen elsewhere or recommended.