But if OpenGL was used, wouldn't that have to go through Zink and MoltenVK for Apple? Or is there a Zink-like OpenGL wrapper for Metal?

SDL will let you push geometry directly without needing OpenGL now with its geometry API, and I think all of its drawing functions use that by default now. I don't know if that's adequate compared to OpenGL but it isn't entirely CPU bound. Dear ImGUI's SDL renderer uses it.

It's a bit more complicated to add the OpenGL renderer, because of subpixel font rendering. You need to sample the framebuffer to do it correctly in all cases, as well as a custom shader. It's on the list of things to do, but as you say, CPUs are generally powerful enough these days; so it's not super high-priority at the moment.

In most cases you should know the background color behind the text without sampling the buffer, or at least get an approximation or revert to grayscale AA when layering text on top of unknown background, but yeah I get the idea and it is an interesting trade off.

Software rendering for a text editor hasn't been a major bottleneck for a few decades, and while resolution has increased CPU speed has outpaced it.