In my experience most metal musicians can hold their own with the best of them when it comes to hard core music nerding with thing like idiosyncratic time signatures and unusual chord and key changes.
Maybe your Berkeley grad metal practitioners like Dream Theater and probably a lot of metal in the last decade or two (since the internet and YouTube) but I imagine a lot of garage metal bands from the 70’s and 80’s were probably just playing/writing by feel and a loose understanding of music theory. I’m sure producers helped bring it all together on the album.
Yea, I think we can roughly split metal into pre-90s and post-90s, with 'modern' metal being a lot musical and precise. Just looking at all the people I know that have been in bands, the more into music theory they are, the more likely they are to be in a metal band.
It’s even more obvious when you consider Metallica’s particular songwriting process at the time: James would mess around making cool riffs, record them into a riff track (just a big pile of his favorite riffs put into one big tape in no particular order), then he and Lars would sit down and listen to it, pick the best ones, and build a song around them.
The 5/8 “stutter” in the verse almost certainly came from the original riff track, and when Lars had to add a drum beat to it, he simply mirrored what James was doing, and that’s how it made its way into the song. It’s not really rocket science.
Maybe based on demos and sheer number of bands we've never heard of. This album in particular certainly was not though. Sweet Silence Studios was a little more state of the art...