This seems to be just one approach to the problem. To my taste, the way they're drawing the vertical lines is ugly (because of the asymmetry), so it made me curious how notable professional isometric pixel art has solved the problem.
I checked two games: Rollercoaster Tycoon 2 and X-COM: UFO Defense. Interestingly, they solve the vertical line problem in the same way as each other, but it's in a different way from the link.
In those games, instead of having an explicit line border, the line is implied by a shading difference between the two faces, which eliminates the asymmetry. Additionally, the corners at top, middle, and bottom are 4 pixels wide instead of 2.
(This strengthens my conviction in the following rule of thumb: when it comes to art, it's better to study professional work I like than to trust a guide.)
This is from the studio "eboy", who's been doing professional isometric pixel work for a good while. When you're a multi-person studio who regularly does sprawling multi-artist projects (https://hello.eboy.com/pool/~Pixorama/1?q=project) then it's well worth documenting what you are choosing to standardize on.
I'm not a huge fan of their signature look either, but this is a nice little list of "what to expect if you're gonna work with us" IMHO. :)
RollerCoaster Tycoon isn't actually "pixel art", but, instead, renderings of 3D models. It just looks like pixel art because of the vibrant palette and small pixel-level art. Pixel art "techniques" do not apply because it wasn't drawn pixel-by-pixel.
Interesting point. That's definitely visible in the overall art style.
But at least regarding the edges and corners of an isometric rectangular cuboid tile, I'm not convinced that there's a meaningful difference between pixel-by-pixel drawing vs non-anti-aliased rendering--the rendering turns out so precisely regular that it might as well have been drawn manually.
I checked two games: Rollercoaster Tycoon 2 and X-COM: UFO Defense. Interestingly, they solve the vertical line problem in the same way as each other, but it's in a different way from the link.
In those games, instead of having an explicit line border, the line is implied by a shading difference between the two faces, which eliminates the asymmetry. Additionally, the corners at top, middle, and bottom are 4 pixels wide instead of 2.
(This strengthens my conviction in the following rule of thumb: when it comes to art, it's better to study professional work I like than to trust a guide.)