Unfortunately this hypothesis is also wrong. MAP_UNINITIALIZED can only be enabled in the kernel when there is no MMU, and in that case the page will already be in physical memory, so the very first pointer dereference will read the correct byte, not a fake zero because it's "uninitialized".
He explicitly states 128byte filename allocates 129 bytes. https://www.youtube.com/watch?v=kPR8h4-qZdk&t=1417s