It was definitely unintentional in TBC back in 2007. Hell, it was probably less of a coding error and more a result of the network infrastructure at the time.
Having some extra delay around seals, specifically going into in TBC Classic in 2021 after the changes to spell batching to allow this, though, was by no means unintentional:
It shouldn’t be, imo, but less-than-intuitive interactions are often considered working as intended, like having Heroic Strike queued up while dual-wielding removes the 19% additional miss chance from your off-hand:
When they showed off the waist runes for Paladin in the SoD trailer with PlatinumWoW, if we’re assuming that only Howling Blast was intended to be a joke, with the other two we saw - Ardent Defender and Empowered Seals - as real runes, we’re not 100% sure how they intend to use them.
Ardent Defender we’ve seen in multiple iterations, but seeing as we have the 20% damage reduction below 35% health baked into Hand of Reckoning, I would guess it’d be Cata-style, an active ability that restores some health if an attack would kill you during that time. (Though with a longer CD.)
Empowered Seals, on the other hand, as it was as the level 100 talent in WoD, doesn’t seem like it would make sense in SoD. Maybe Empowered Seals would just be a name they use for something that will allow seal twisting. I don’t know, just spitballing.