If that was confirmed during Sinnoh, it's pretty much become lost and overwritten by the sudden developments without much explanation in the following sagas. I'm pretty sure the writers don't expect people to think that far back without bringing it up when it should (hence why Ash may re-scan Pokemon he had already seen before).
Not really. It was also referenced indirectly in Kalos. We see Siebold and Malva both referred to as Elite four Members and Diantha as the Kalos Champion. Claiming Kalos Champion is < Elite four just doesn't make sense considering the fact that this simple concept has been reinforced every single series of the anime beforehand, including the games. It's not as complex of a detail or concept.
This is one of those things that the show does to help introduce (or remind the audience about) a Pokemon and sell merch. It‘s solely for the benefit of the audience, especially the target age brackets that are new to Pokemon and may not have played the games or watched previous series or may not have even been conceived when the Pokemon first showed up onscreen.
There are a lot of things that the games and the anime have yet to confirm. What is the regional Champions League and how does it work? Who is in it? How does someone become a member of the Elite 4? Are the Elite 4 members just those that reach the Semi-Finals? Does it work like the tournament in Sw/Sh?
I always wondered why we ended up with Victory Road and the Elite 4 in the games (prior to Sw/Sh) instead of a proper tournament.
Yes I definitely agree but the concept of "Elite Four" & "Regional Champion" has always been the same for every series and characters have been referred to accordingly.
In Kanto Bruno is referred to as an Elite four Member, in Hoenn we have Drake (Who is referred to as an Elite four member), In Sinnoh we see all the Elite four members and Cynthia (Referred to as The Sinnoh Champion), In Unova we see Caitlin referred to as an Elite four Member and Cynthia/Alder referred to as the Sinnoh/Unova Champion, in Kalos we see Siebold and Malva referred to as Elite four members and Diantha referred to as the Regional Champion. Even Steven is referred to as the regional champion of Hoenn here.
And Finally in Journeys, we see Wikstrom referred to as an Elite four member and Iris referred to as the Unova Champion.
So considering the consistency so far across the series, claiming Iris is not Regional Champion tier just because she logically shouldn't be based on her performance against Ash and how she stacked up to Ash before in Unova.
The only point it reinforces is something that has already been reinforced multiple times across the show, i.e Pokemon battles are 100% driven by plot and rarely have any logic/ consistencies in strength.