think of it as the split between a browser engine (renders and handles interactions with webpages) and a browser (provides the user interface and everything other than rendering the webpage, e.g. bookmarks, tabs, clipboard interaction, etc.)

similarly, a terminal conceptually consists of a terminal engine, which handles text rendering and interaction, and then a terminal which handles tabs, splits, sessions, etc.

now one key point is that you can get a terminal engine right according to a bunch of reasonably objective criteria, so it's a problem well worth solving once and for all. the terminal itself is a lot more subjective and idiosyncratic; different people want different things out of their terminals and the reason is very often that it fits the way they work, or they enjoy some particular features, and even the absence of other features. it is wonderful that libghostty can solve the part that everyone agrees needs to be objectively correct, and lets people concentrate on building the subjective part in the way they like best.