I could be wrong, but I'm pretty sure the whole "Dio is a heavy metal king and synonymous with heavy metal" thing was more of a latter thing, helped by Tenacious D and other metal heads looking back with hindsight.

But I think during the 80s he was just another one of the metal guys, who had some hits, like Holy Diver, Rainbow in the Dark and Heaven and Hell (with Black Sabbath). I don't think Dio would have been thought of as "The personification of metal" anymore than Slayer, Metallica, Judas Priest or Iron Maiden were, and maybe less so than those examples.

And yet if you listen to his music and his vocal style, it's almost like "the most metal-y metal that could ever be metal-ed".

It's so "metal-y" that it almost looks like when fiction tries to portray something real and comes up with something that is a bit over the top and doesn't ring exactly true. He even sings about magic and dragons and all that stuff. Ronnie James Dio, with all his dungeon and dragons type lyrics, is the metal guy a hipster in the 2010s would create as a movie character in a movie set in the 80s as the token "metal guy".

And yet, unless I'm wrong, he was just another one of the metal guys back then, and it's more a modern thing of Dio being this metal royalty and synonymous with it visually and in sound.

I just think it's interesting he wasn't bigger at the time than he was given how much he fits what we think of the traditional 80s style metal, compared to other bands and artists.