https://www.warhammer-community.com/2017/07/05/codexes-your-questions-answered-july-5gw-homepage-post-2/
That's because it wasn't in an Errata or
faq, it was on one of their actual pages where they were just answering questions in general about indexes and codexes.
This however has led to quite a few different interpretations of the rules. Some say that you can only use index entrys if there is no entry in the codex for it (example : tech marine on a bike) but if the same entry is in both (example : librarian in term armor) you have to use the most up to date one and not the index option.
Others say that you can use either entry but have to pay the costs from the index if you choose the index for the entry (example : if you pick a lib in term armor and want to give it a powerfist for some reason you would then pay the index cost of a powerfist not the codex). I prefer this option personally because it shows a bit of a tax using these old designs and rules that do not show up anymore but still let's us old timers play our units fairly if we want.
Finally there is a group that says you can use index units but pay codex prices if you want. Not a lot of Support on that one but it does come up.
You will want to talk to your opponents and work out how you would expect these rules to play before gaming. It will solve more problems down the road as more codexes come out.