The diegetic-ness of spells is pretty arbitrary, though. It's just "magic is unquestionable" by habit, not by strength of explanation. Given that "magic" is just whatever the game needs it to be, and has no consistent or coherent standards to which it must adhere, claiming magic is diegetic is just saying that "what I make believe is fine is fine."
Spells are the acceptable metacurrency of D&D, with entire classes built around the concept of "I get to dictate fiction in discrete chunks," while any other arrangement of such is viewed as unacceptable and wrong and gets labeled as non-diegetic when there's ample diegetic descriptions available that have at least as much explanatory power as "but magic."
Let's be honest, there's are lots of metagame aspects (spells, hitpoints, AC) that are grandfathered in as acceptable, it's just new applications that get the sideways looks.