It really isn't; this doesn't change anything, we previously knew we were at least 200k years old, that fact is still true. We now know we're at least 300k years old, that fact will also always remain true. What's at fault is your understanding of what science is saying rather than what it's actually saying.
> that I can help wondering why we speak so confidently about the origins of our species
Because it's a given that we're always speaking about current best available knowledge. What is proven true remains true, it's just refined over time as new truths are discovered. What we know continually grows, it's not like we're changing what we know to be something else, we're just growing what we know continually.