They're not asking you to re-implement it on a whiteboard so they can open a PR and shove it in their new app. They're asking you to re-implement it to demonstr

They're not asking you to re-implement it on a whiteboard so they can open a PR and shove it in their new app. They're asking you to re-implement it to demonstrate that you understand it, and that given an understanding of something, that you have the skills to implement it. If you can't hammer out a technical interview using off-the-top of your knowledge, you're in the group they're trying to weed out.