Porcelain bathware (sinks, baths, toilets) and sewer plumbing is a perfect metaphor for the distinction between UX and underlying infrastructure. Despite the simplicity to which they are connected together, one is a near-universal experience and the other is a massive layer cake of specialist engineering that very few laypeople understand.
It's called git, and Linus knew exactly what he was doing when he named it. I doubt he cares a jot if the plumbing/porcelain metaphor icks you out a bit.
And even more meaningfully for tool authors, the plumbing is very long-lived and general-purpose, with basic objectives like not leaking and not breaking down, while the porcelain is rather easily replaceable, possibly specialized, "smaller" but more complex than plumbing, with meaningful user interface (in Git's case, specifically, designing commands that are unlikely to do harm, easy to understand and efficient after they have been learned) and significant dependence on tastes and environments.
No, plumbing isn't a bad thing. As I generic term I might use it myself. The porcellain part though moves it clearly to the toilet topic, and that part I find unfitting. And the original poster just didn't understand what was meant with the differentiation between the porcellain and the plumbing. Which caused me to comment that software authors should be a bit more carful when naming things. What starts as a inside joke might not be well understandable by outsiders and off-putting in the extreme cases. After all, naming is about being understood.
I think this terminology was not well thought through. It's somewhat degrading.