I guess for the large part the inter-op here is from (mostly) interpreted languages, which are certainly higher level than C++.
Maybe instead of it being a binary choice between high-level and low-level languages, there is a middle ground. Fairly easy to argue that as the case.
After reading the first sentence on the site, though, I think this was just a somewhat clumsy rephrasing. "SWIG is a software development tool that connects programs written in C and C++ with a variety of high-level programming languages." "A variety of high-level programming languages" implies there are some high-level languages that SWIG cannot connect C/++ with, and it's open to interpretation whether C/++ are among the high-level languages that SWIG doesn't connect C/++ with.
There is: mid-level languages. I was taught (and still think of) C as a mid-level language; things like assembly are low-level languages; C++, Python, Java, etc., are all high-level languages.
But I'm a graybeard, and these are the meanings that I've learned. It's possible that the definitions have shifted and nobody told me.