Yes, those are frontend developers, we all agree with that.
Should every frontend developer be reinventing date pickers from scratch just to demonstrate how good they are? I don't think so.
That's why I said 99%. Maybe there's an 1% or even less where it makes sense to reinvent it because you're Airbnb or Amazon or Google or you are in the date pickers business.
But yes, you are still a frontend developer if you don't do your own datepicker and focus on shipping your company product. And date pickers are built by frontend developers. What's your point?