Earlier quoted context omitted.
It looks like GitHub Discussions is sort of filling a similar niche--I'm guessing they're banking on being able to build that out for less than $1B--since they already have GitHub, they don't benefit as much from the StackOverflow user network / brand. I think that makes sense.
Easy to build out the platform, difficult to build out the knowledge base (especially since so many areas have already been covered on SO)
What matters more is the rate at which content is generated, and I think GH has a pretty competitive story considering that people naturally want to ask the project maintainers/community questions directly (as evidenced by all of the GitHub Issues which are formulated as questions) and considering GitHub's significant share of those maintainers/communities. I posit that new content will increasingly appear on GH Discussions.
Please don't mistake this for some exaggerated "GH is going to kill SO" argument--SO will do fine, but there is a compelling story for why MS wouldn't spend $1bn on SO.