Earlier quoted context omitted.
That's because most aren't designed as git porcelains. Magit enhances and, in some cases, replaces git's abstractions with its own. If a git frontend starts by taking git's own built-in porcelain as a given then it's already doomed to failure.
I've been using Magit for years, but I've never seen the use of "porcelain" as a jargon term to describe anything but Magit. Can you point me to a 'splaner on the use of that term? Googling hasn't gotten me anything but an insight to just how many people take porcelain (the pottery) very, very seriously.
What the parent mean is that some tool rely on the human-oriented, potentially unstable output of git commands instead of using lower level layers.