The author's point is to use an immutable or atomic system. Debian uses a traditional package manager and over time those packages can get in a weird state. Battery runs out during package update? Third party repo disappears? GPG key changes? Distribution eliminates 32-bit library support? Nvidia drivers change? Distribution deprecates Xorg? Your everyday user doesn't know how to deal with that.
An A/B partition scheme is very simple and the system as a whole is integration tested in CI. An OSTree based atomic system is pretty close, although there is some added complexity.
Not to mention Debian out-of-the-box is a pretty miserable experience for everyday users. Their Nvidia drivers won't work, they can't figure out how to install Chrome, videos won't play due to missing codecs, etc.