I think they're meaning is that it's the engineering that's "lost," not the technology. Most of the "technologies" of the Saturn V are around. But something to replace it has to be re-engineered/re-designed from scratch, which is massive considering how there are plenty of other things from that era that have been subject to incremental and generational improvements continuously. If someone were to make a replica with modern technology every little gotcha that got worked out and worked around in the original would have to be re-discovered and re-worked around.