What is misleading about the title? A custom encoding scheme is exactly what it suggests. Maybe it has been edited since your comment was posted?
I'd actually point at the other half of the title than the existing comment when being pedantic "Replacing [...] with a 64-Bit Word" isn't quite right, it was replaced with a manually packed 64-Bit Word and the occasional heap allocation.
I'm not sure being this pedantic is particularly useful in titles though...
It wasn’t replacing one rust enum, it was replacing what are effectively multiple enums