Again, this is something that's often repeated all over the internet, but there is no source for this, it's just speculation - and fairly unconvincing speculation at that, since it has to go so far in assigning the designers these strong opinions and unwillingness to compromise just for it all to make sense. I feel like what I proposed is a far simpler and more straightforward explanation. Occam's razor and all. Just look at what the mouse looked like through its generations[1]. When redesigning it, they obviously just took out the single-use battery compartment and replaced it with a section containing the rechargeable battery and the charging port. In fact, they really couldn't have done it any other way, because the mouse is so flat that its top and bottom sides taper all the way to the desk, with no space for a charging port. So, when making the gen 2 model, just putting the port where it is was probably a far simpler drop-in solution that saved them from having to redesign the rest of the mouse.

[1] https://cdn.shopify.com/s/files/1/0572/5788/5832/files/magic...