Question about garbage collection specification

[rsw0x via Digitalmars-d]

http://dlang.org/garbage.html

Do not take advantage of alignment of pointers to store bit flags 
in the low order bits:
p = cast(void*)(cast(int)p | 1);  // error: undefined behavior

if this restriction is actually imposed - why does 
std.bitmanip.tagged{ClassRef,Pointer} even exist?