I don't think it is great question generally but this also depends on the position, the level of technical aptitude and seniority expected.

It is not something I would expect most people to rock up with in 15' while someone is looking over their shoulder. Probably it is a question that will help me to distinguish a kick-ass junior than someone who has a standard Keras syntax understanding but for more seniors roles this is likely a bad indicator. Far less senior engineer tasks fail because the person couldn't code backprop from scratch than because the wrong architecture was chosen, or they didn't know what part of an existing pipeline to optimise, or where to look for potential bugs given a particular unexpected behaviour, etc.

In general, I think it was more a point of "showing your thought process" than actually getting the code right. I would "abstract" things quite a bit first and then "start coding". But as others said, if they absolutely need to "split hairs" that is a way to do it too.

Best of luck with your interview in any case!