Not true, in at least 99% of the cases. Perhaps even 100% if a bug that ahppened sparingly over a year ago is now fixed.
When an arena run is retired, such as when Rise of Shadows was released, the prices will be added. At least most of the time, probably all the time, the pack is added as an unopened pack. And it then goes unnoticed by the player who is busy opening the packs from the pre-release bundle and from the free promotion events that featured before and after release. They don’t notice that they open one more pack than they should have - that pack is the Arena pack.