I don’t think the keyboard is any more broken than it has ever been. It works pretty well for me aside from its awful, awful repeated "corrections" it applies, I delete and it reapplies. This is not new at all.
There are a lot of broken things in iOS, just try any apps in landscape and you'll wonder if QA even realizes the iPhone has landscape.
I know the video, again it doesn't matter. I speed type on my phone exactly the same way I used to. I understand how autocorrect works and that it retroactively fixes multiple words later on. I typed this whole comment on my iPhone and I encountered no such issue.
What the video shows could be a problem only if you disable autocorrect or if you want to type a single letter. I made a bunch of errors typing this one and in the end the phone autocorrected it all.
There are a lot of broken things in iOS, just try any apps in landscape and you'll wonder if QA even realizes the iPhone has landscape.