It's been a while since I've done actual development, but determining tabindex ordering was a very big deal in the early 2000s when clerks/admins were moving from legacy systems to the web. They did most of their jobs with a keyboard, and if you set up tabindexing so that they can use a keyboard for 90% of their job then they would fly through tasks. If you didn't then you'd have some very angry clients at the end of the project ;)

So just echoing that this is not just an accessibility thing, but just a good user interface design thing. If your UX designer isn't taking keyboards into account (jettison mobile first development into the sun please) then they have no business building business applications.