I think the issue is that many of us are poor communicators (myself being number 1) so we say nothing I can do will help you instead of I can't teach linear algebra, numerical analysis and do the code review in an hour. Therefore putting a junior engineer who has a weak math background on a code review for my new algorithm which uses spectral sparsification for approximating matrix inverses in n log(n) isn't a good idea.
But a junior can still ask question like: "why isn't there a test for this code", "why are the business logic and algorithm code mixed together" or "why doesn't this align with the design documentation we wrote last week, does it need updating?".
And the junior will learn which things to learn. I certainly think you should allow juniors to code review senior coders work if for nothing more than mentoring, but I am sensitive as to why an overloaded senior would balk at the idea.
