Oh, I have no doubt that they could have extracted those gains from Zig! My point is more that, from a relatively naive line-to-line port, they were able to claim these benefits without much effort.
It's not great for Zig if you have to put in more work to end up at the same place efficiency-wise, especially for a language marketed at people who like to get the most out of their metal.
That's not what the article says though. The size reduction was from extra linker flag for deduping code, and the speed gains from LTO.
They could have done the same in Zig, even though it probably shows cargo is better at this than build.zig.