There's the current style guides and then there's the history (well my hashed together version from a bunch of reading and podcasts)...
Porter became the common beer style in England in the 19th century, having started a fair while earlier. It was the everyman beer, a beer for the porters (those emptying the ships of all the wealth coming from the colonies). It was so popular it was also exported all over the world. A bunch of trends pushed people away from the standard dry, average ABV porter including...
Some pubs would do a different take on the common porter. It'd be sweeter, more alcholic, more smokey, whatever, but in some dimension it would be stronger. And these would be called a stout porter.
Shipping beer to the colonies led to putting more alochol and hops in the recipe to stop infections that could give you the inconvenience of an exploding barrel maybe blowing a hole in your ship (or at least covering everything in beer). Most of these beers ended up with names other than porter, like tropical stout, baltic porter and russian imperial stout (though RIS is a lot more complicated and full of myth than that).
Malting technology got better and brewers had access to decently modified grains that were pretty pale.
So those and probably a bunch more factors, mean that the traditional dry, average ABV porter is relegated to an old man's beer in the first half of the 20th century and finally dies out in the 60s/70s. Then as part of formalising home brewing comps in the US in the 80s the style is included and that gives it the impetus to kick back up as if it had never gone out of fashion.
So I'm sure there's a fair bit of romance obscuring the facts in that version (especially the tax and economic realities that are generally more important than expected), but that's my current understanding.
edit: so I'm guessing Guiness goes with the stout styling because they've got a fair amount of the really roasted malts to make it stout in that dimension and to stay popular (avoiding that old man's beer style stigma).