I suppose it may be an implementation detail, though I wouldn't be amazed that had enum existed when re was written it'd have been used instead of constant integers at the time. Though I do suppose exposing it fully would add two ways to get the flags which I can see how it would be considered bad.

It's still useful for type checking though, and while I did make a PR to add it to typeshed, that still leaves it in an iffy state and I probably would not be the last person to be confused by it initially.