As the title implies, their sig is:
"pure nothrow @safe int btc(size_t* p, size_t bitnum);"
The argument "bitnum" clearly implies array indexing. In this case, there is no bounds checking, which means the function can't be safe.
Furthermore, they access data in an underhand fashion, potentially corrupting anything that is passed to them.
Strangely enough, the functions "bt", *is* @system.
In any case, they cannot be made @safe in any way shape or form.