Truly seamless because the zig compiler is also a C compiler, so the type information and calling convention works across languages at a level above any other I've encountered.
True, but I think that the person you're responding argued that rust/C integration is also seamless. (In the general discussion I'd say they're right as C to Rust integration isn't much of an problem and you can use C libraries relatively easily in Rust as well, but at the same time when talking about Zig I don't think it's fair to put it on the same ground).
It's also an unfinished language. I agree Zig is promising, but it's not confidence inspiring when the creator is still making videos debugging the compiler.
It is impossible to do C interop without a C compiler, by the way.