He clearly means that he used two languages: C and C++.
"I decided to reimplement the inference of the model from scratch using C/C++. To achieve this I implemented a minimalistic tensor library in C and ported the high-level architecture of the model in C++."
It is obvious what he did just from reading what he wrote, after. It is not clear that he meant anything at all by the expression. You cannot use his thing in any but a C++ program, so mentioning C at all only adds confusion.
implemented a minimalistic tensor library in C
ported the high-level architecture of the model in C++
The C part - which accounts for most of the lines of code - may be of interest to someone who wants to use it as the foundation for implementing a different model - not necessarily in C++
He clearly means that he used two languages: C and C++.
"I decided to reimplement the inference of the model from scratch using C/C++. To achieve this I implemented a minimalistic tensor library in C and ported the high-level architecture of the model in C++."