Yes, you need the SM
codex plus the BA
In former editions, BA
had a stand-alone codex but these days are gone.
had to share a codex with the DA
, I think it was called Angels of Death back in second. They were also just a supplement booklet to the main SM
codex in 3rd or 4th edition.
Really they have just gone back to how they used to be with the other 5 first foundings also getting a bit of extra love (to be honest up until 8th it was Codex Ultramarines in all but name like it was back in Second)