What he actually said was, "The byte order for 8052 processors depends on the used compiler." (my emphasis)

Because the 8051 is an 8-bit processor, it knows nothing about multi-byte data at all^[1] - therefore the compiler is completely at liberty to do whatever it likes.

This is not the case, however, with >8-bit processors: in this case, the processor hardware defines the order in which it expects the individual bytes of its 16-bit, 32-bit, or whatever-size words - and the compiler must abide by this.


[1] Apart from the odd 16-bit SFR, like the DPTR...