I'd guess it would be similar to (if not the same as) the internal pipe + select() approach mentioned in the post (although most likely with epoll or whatever instead of select).
Yeah, it seems like someone heard 'propagation' and assumed a push-based system, then found out that was wrong, which means either they were wrong to assume propagation==push, or 'propagation' was the wrong term.
So instead of re-examining their assumption and I dunno, googling "what does propagation mean" or whatever, they've decided the problem was that ~everyone else is wrong...
Yeah the article is pretty terrible IMO. It seems to be pitched at folks who don't know any better, so just showing that you can use 'source' like a kind of config parser without pointing out that it totally just executes code is pretty reckless.
I thought it was going to be something more like "here's a way to read KEY=VALUE lines into an associative array" (which is pretty easy, and can lead to all sorts of fun discussions/learnings about arrays in bash, subshells, whitespace/$IFS, etc)
Shame, if you'd kept reading you would've been enlightened by the author's shocking discovery that modern computers can solve problems orders of magnitude faster than computers from the 60s or 90s. Just mind-blowing stuff, who'da think it