The article says "Also, because ARRI’s earlier color space, AWG3, is larger than ACES AP0, original ALEXA footage sometimes clips in the blue region when mapped into ACES. " using these side-by-side images:
These images are taken from this article on Aces Central.
which says at the very top “ACES 1.3 introduces a new and improved solution to this issue with a Gamut Compression LMT.”
That this is not mentioned in the article strikes me as a notable oversight.