I tried controlnet in a few forms with some lettering, but wasn't able to get close to how they're doing it. Too often it will generate the subject matter into the background too. Firefly's has really good separation usually. Maybe they're doing some kind of latent coupling, where the letter is a masked area and the prompt is applying purely to that masked area.

I think those are fair critiques. I also noticed that it's a transparent PNG that doesn't look great on every background.

I wouldn't want them as vectors fonts necessarily. One of my complaints was that duplicate letters would just re-use the texture. I get that it's probably a caching mechanism to finish generating the word faster. But, since the AI can generate infinitely, it seems a bit self-limiting to re-use a letter. I guess my complaint is that the re-use really points out that it's _not_ a custom illustration.