The only justification for not using fixed static values here (ala enums - an enumeration of possible values) is the potential case of having values that are dy

The only justification for not using fixed static values here (ala enums - an enumeration of possible values) is the potential case of having values that are dynamic at run-time, rather than design-time. Being as that the article consistently refers to these being fixed values, and that they're optimising for 3rd party use of their library, that doesn't seem to be the case. This seems to be the classic case of a solution looking for a problem.