It's moreso that a backend developer can now throw together a frontend and vice-versa without relying on a team member or needing to set aside time to internalize all the necessary concepts to just make that other part of the system work. I imagine even a full-stack developer will find benefits.
Copilot is going to feel "amazing" at helping you quickly work within just about any subject that you're not already an expert in.
Whether or not a general purpose foundation model for coding is trained on more backend or frontend code is largely irrelevant in this specific context.
