I’d say that what you mentioned is more about the screen width than the aspect ratio.
Framework decided to make very efficient use of laptop space by making the display very similar in width to the keyboard, that way there doesn’t have to be much wasted space to the side of either. Likewise they decided to make the display similar in height to a keyboard with a decent sized trackpad below it. That happens to be approximately a 3:2 aspect ratio.
By comparison my 14" 16:9 Thinkpad (which actually has 0.4 in² less screen space than the Framework 13) has meaningful wasted space both on the sides of the keyboard and below the display.
In their announcement What’s new in this year’s Framework Laptop 13: Part 1 (upgrades) Framework stated that:
“we worked with our supplier ATL to figure out how to maximize capacity within the same physical form factor as our original 55Wh battery. We landed on getting 11% higher capacity out of a new lithium ion chemistry with higher energy density. This includes moving to a new 4.45V cell voltage.”
They also included a graphic indicating they still expect it to maintains 80% capacity after 1000 cycles.