So they're definitely in the space (I just read a fantastic rust post on there[0] today, thanks to This week in rust[1]) but you can't beat the staying power/massive bags of money/developer goodwill that is already heaped on Github. Github entering this space would be a death sentence for a site like dev.to, unless they massively fuck it up (or dev.to adapts and provides some extra value-add/integration).
imho it looks like dev.to is a cross between twitter and a full blog (seeing it for first time today). Github readme looks more like a magazine where the content is curated.
The difference between user generated content and curated content is a big enough differentiator that I don't think it'd be a death sentence.
[0]: https://dev.to/pnehrer/a-story-of-rusty-containers-queues-an...
[1]: https://this-week-in-rust.org/blog/2020/08/26/this-week-in-r...