If you read the original referenced article / blog post, one of the shortcomings presented is lack of C++ interop in Swift. If you are asking where is a good C++ library for advanced arithmetic, I would recommend here
Boost adds some nice math support but also in general adds immense value to C++ and by necessity isn't easily back-ported to pure C.
What is in C++ that makes math easier is some of the core data structures such as STL 'map' and 'multimap' which is an ordered associative data structure, similar to a native implementation of an ordered dictionary, which is a nigh impossible to find data structure in most languages (usually internally implemented as a red-black balanced binary tree). I have also been advocating for a native Swift implementation of ordered dictionary to be added directly to the language. It isn't in the Objective-C or C spec. 3rd party Swift libraries do exist for ordered dictionary but it needs to be part of the Swift core or the Swift standard library (1st party).
For native Swift, even as a 3rd party library I don't know of anything like ~dhbailey's ARPREC or QD. If I am wrong let me know. For advanced math there is alot of stuff out there in C++, not as much in pure C. We are expected to on our own backport these libraries to pure C or potentially more arduous port them to native Swift. I need a little bit more help from the language itself. I've thought several times of attempting a manual port of QD to Swift but the language keeps changing enough every year that it discourages me. Put some more of this stuff in the language directly, or add interop support for native C++ to Swift, or I have to wait a few more years for Swift to be less churny and then I will attempt the port myself.
I have previously ported red-black balanced binary tree to Swift (back in 2015) but then the next version of Swift came out and it broke everything and I gave up at that point. If the implementation is a core part of the language or part of the Swift standard library then Apple has to deal with the churn of their own language breaking their library, instead of being maintained by a solitary person outside the company.