People can dismiss language ideas and concepts that "have been known and used for many decades" just the same.
For a trivial example: significant whitespace.
So, e.g. "Lisp has had that since forever" is not a guarantee people would be OK with it, unless they also like Lisp - not to mention they might not even know about it in Lisp.
As for C++, many people consider templates - at least as implemented - a bad idea. So some people knowing that it existed in C++ already, and considering it a bad idea to have in Zig is not contradictory.