Based upon "Hayyei Qehat", he proves that 400/430 years in Egypt cannot be taken literally - despite explicit texts saying so.
Literalism is set aside here by Rashi when logical deduction of other passages makes it difficult or impossible to a literal read
Then the issue remains what to do with the 400/430 years!?
It seems to me Rashi could have gone the other way. That is why not say the Yichus was not literal instead? And that generations were skipped and the number of years was literal!
The response to that is that Hazal have deemed that period as 210 years. It has been adopted AFAIK by Seder Olam
This same issue is in the current NishmaBlog poll re: may we set aside a literal read of text when Hazal themselves have not chosen to do so?