Is the Armoury System A Sign of [Spoiler Alert]

[Enkidoh]

A 1.0 GLA class quest cutscene outright showed a NPC fighting in the Colleseum switching class mid battle from GLA to MRD (story wise he was actually called out for basically cheating in the fight as swapping class mid-fight in the arena was a big no-no, it didn't help him get an edge though hilariously, as his GLA opponent still kicked his butt ), so swapping class is done by others in-universe - SE themelves way back in 1.0 outright termed it as simply changing to the required weapon or tool.

Now instantly swapping all your armour though at the same time, that is gameplay and story segregation (Derplander doing that in the SHB's opening is just rule of cool - note even the sound effect is the same as changing to a saved gear set in-game in ARR when he swaps class in the FMV), as when he was shown swapping from WAR to DRG at the end of the Heavensward opening to join the battle of Ishgard, he's shown manually removing his WAR AF1 gauntlets (which fall noisly to the floor) and then setting his DRG AF2 ones on his arms all badass-like as he walks to the Steps of Faith (as Rannie above mentioned), which I think is a much more realistic depiction of how characters change equipment in-universe.

But like I said, rule of cool applies.