Would you expect a senior software engineer how to implement a specific algorithm in language x?
Yes. Hell yes. Never mind senior software engineer, I’d expect that of a junior programmer.
I wouldn’t expect someone to implement a flawless C++ quicksort on the spot, but I’d certainly expect them to be able to sketch out a decent sort algorithm (any of quick, heap, shell or merge sort), and know that it’s better than bubble sort and why. I don’t expect programmers to be writing their own sort routines very often - if they’re doing that, there’s something wrong - but I do expect them to know how sorting works and what the costs are, that there are different ways to sort data, that the different algorithms have different costs and characteristics.
If you can’t do that, you’re the problem. If you don’t think it’s necessary, you’re even more the problem.