kTextAlignment is currently defined in CoreLibs/graphics.lua, as a global. I wonder why it wasn't defined inside playdate.graphics like all other constants?
This is mostly just a cosmetic issue when viewing Lua Memory in the simulator, where kTextAlignment seemed out of place.
I don't think there is a well-thought out reason for it, and you're probably right that it should be part of playdate.graphics.
However, it's probably too late to change it now without requiring code changes for existing games (or adding duplicate variables which I don't really want to do) so I guess it's just a fun little quirk of the SDK?