I never thought about it that way because I've never really seen armor as an ability per se (with exception Trollolo). That's why I always expected the game to behave like it does right now. But I do understand how this description can be seen as misleading!
I think it would make sense to either change the wording for armor a bit (and not call it deploy - maybe just name it "protection: 5 armor" or something like that) oder really change the order of events and let cards like Stennis get hit before the armor is added.
Morenn's attack triggers before earth elemental's shield goes up when that card is played with the words: "deploy, give this unit a shield" which is identical to Stennis's text order, therefore this is indeed a bug IMO.
In one of today's many games, I had a Morenn down as my opponent's Alba Pikeman played a 2nd Alba Pikeman from deck.
If I understood the intended interaction correctly, Morenn's ability does 5 damage to the Pikeman BEFORE its deploy ability comes into play. In this case, that would mean that it would be dealt 5 damage, killing it, before it gained it's two armour, as the armour gain is specified as a deploy ability.
What actually happened was it came into play, seemingly with the armour already on it, meaning that he was reduces to 1 health, and proceeded to happily summon friends ...
I haven't had a chance to test this with other armour gain on deploy effects, but may be the case with them as well.