I think both sides of the argument are being overstated here. The truth, as happens so often, lies somewhere (IMO) in the middle. To use your analogy, whereas it shouldn't be a go-nogo criteria to have written a language, I remember that the first time I did it it completely transformed my coding style from single diatribes with lots of ifs to tight multi-level routines of one or two lines each. It meant that instead of development and testing for periods of arbitrary duration I could suddenly write high quality work to a deadline.
So yes, I for one would be influenced by that in a CV, though of course there are more usually more direct criteria relating to the job in hand that are going to take precedence more often than not. So yes, I agree with the OP insofar as it is a huge advantage to have done something from scratch - modules and other tools can often replace effort, but they can't replace knowledge and experience.
^M Free your mind!