Writing that programmers that use Euler instead of RK4 are "bloddy idiots" might work well to get some laughs in a blog post when the author is trying to stress how RK4 is more accurate and stable than the alternative, but there are cases where real-time applications (especially on older machines and older consoles) could not afford the overhead of RK4 and Euler was gave good-enough for what they needed.

Most gamedevs haven't really researched numerical integration, they've just heard that the naive method, which is very often hand rolled outside of physics engines, is Euler; and that particle simulations should use Verlet because reasons. (The inner reason being that the first Hitman game used Verlet and they published a paper about it.)

Using irregular timesteps _might_ not be a bad idea if you know what you are doing (but probably not for the usual kinematic simulations). See Dynamic Monte Carlo, Gillespie algorithm, First Reaction method etc (these are mostly the same thing).

The most common integration method in physics engines for games is semi-implicit Euler, which the article is implying from the source code. I think you are confused with explicit (forward) Euler, which nobody really uses.