That number itself is not saying much.
Let's say I have an academic article written in Word (yeah, I hear some fields do it like that). I get feedback, change 5 sentences, save the file. Then 20k of the new file differ from the old file. But the change I did was only 30 words, so maybe 200 bytes. Does that mean that Word wrote 99% of that update? Hardly.
Or in C: I write a few functions in which my old-school IDE did the indentation and automatic insertion of closing curly braces. Would I say that the IDE wrote part of the code?
Of course the AI supplied code is more than my two examples, but claiming that some tool wrote 70% "of the code" suggests a linear utility of the code which is just not representing reality very well.