No, allocators are not magic (in that regard) in C. There is nothing out of the ordinary going on with an allocator, the parent comment is simply mistaken (as pointed out by another answer).
Ah, I see that it's because the char type never violates Strict Aliasing. I was wondering how you could define a type such as Chunk, yet hand out a pointer to it to a user who will cast it to some other type.
Well the pool code fails to use char* type to write inside block of memory.
If you look at 'pool_free' code you can see that it receives used block from user as 'ptr' then casts it to 'Chunk pointer' and writes to into Chunk member value of type 'Chunk pointer'. I had to change that line to memcpy when I did it last time around 15 years ago in C++ with GCC 4.something. In short if you are writing your own allocators you need either to disable strict aliasing like Linux does or do the dance of 'memcpy' when you access memory as 'internal allocator structures' right after it was used as user type. When it happened to me write was reordered and I observed code that writes into Chunk* next executed first and write of zeros into user type second.
Note that C++ has different rules than C. C has type changing stores that do not require memcpy for allocated storage (i.e. no declared type). Older compilers have plenty of bugs related to TBAA though. Newer versions of GCC should be ok.
