I'm fairly sure the method he's using is modeled on the you tuber I mentioned. I haven't grown in Coco but the guy I'm talking about treats Coco like soil. I'm not saying you are wrong, I'm just saying if he's been treating it like soil based on someone else's method, it only makes sense to find a solution based on the same method. The you tuber I mentioned has a lot of controversy around his methods as well. with some going as far as saying his entire method is bogus and it's impossible to grow like that successfully. I haven't done it my self so I make no claims either way. I'm just sharing what I see from others working with the same things.Imho its best to treat cocos like hydro and not like soil so I use a 5.8 PH when Im in cocos..
In this case not much of a difference since he will be giving only plain water from here on out or maybe only a little PK at this point in time.
Autoflowers will also yellow by now and its normal. They do that also if there are no lockouts.