> This is a great example of how ADTs can be implemented in C by emulating classes, despite the loss in brevity.

I don't see it that way, mostly because ADTs don't require automatic destructors or GC, etc, but also because I never considered a unique/shared pointer type to be an abstract data type

> When you free the allocator, all pages are freed at once. Not only is this less error prone, but it can decrease performance.

How does it decrease performance? My experience with arenas is that they increase performance at the cost of a little extra memory usage.