Im the creator of HyperTurtle ROM for A9+ / S100. I have seen the work of santo rock from a21 xperia, i was inspired that i can also do the same. So i start from scratch, use base ROM of S100. No i did not port it, because the end product is so different. If it was a port, it will be replica.
Why are you porting our work? Cant you create your own? ROM making not only involves compiling / decompiling, adding tweaks etc etc. It also involves creativity which makes a ROM unique. I am flattered that you also choose to port my ROM, is it that popular?
Last, a21 and a9 ROMs are interchangeable. Just use correct boot.img and it should be ok. So i think there is really no porting going on.