Ignoring Cas’s blunt and completely hilarious sense of dry humor for a moment, think about what he’s actually saying here. Castiel is the name given to him by God. Castiel translates roughly to ‘my cover is God’ or ‘shield of God’ in Biblical theophory—the ‘el’ suffix means ‘God’, and ‘iel’ means ‘of God.’ Cas is the name given to him by Dean. Deliberately or not, Dean removed the part of Cas’s name that means ‘of God’, and left him with ‘shield’. Castiel isn’t actually a Biblical angel—it’s a variant of the name ‘Cassiel’, who was an archangel in the Kabbalah responsible for observing the Earth with no interference. Making it up as we go, indeed.
Remember as well, as Naomi pointed out… Castiel has been reprogrammed so many times… he is not aware of his own history in huge chunks. An Archangel who had a crack in his chassis… who had his ‘mind’ scrubbed clean and reprogrammed… Metatron sure acted like he knew him more intimately then ‘just one of those other angels’. God brought Castiel back… kept him on the map… even when he didn’t do it for His other children like Gabriel, Michael (really could on and on with that list). And why did Metatron need Castiel’s grace specifically? I mean, yeah sure… we all have our own headcanon about it but… could it also be because he needed the grace of an Archangel? Which is why he didn’t take Naomi’s? Do you think perhaps when Castiel was made ‘new and improved’ it was God giving Castiel back his Archangel status to make up for not being there? Just a random dribble from my headspace that would make for a good fic lol.