> You can think of this as subtracting older from yours and adding the result to mine, or as merging into mine the changes that would turn older into yours.
The point stands: taking history in account is unconvincing, as it assumes intent where none is explicitly given. In the example, if the top branch had a single commit, AB => ABGAB, then there is no way to reliably infer that A 'intended' [+ABG]AB, vs. AB[+GAB]. Even with the given history, maybe the author of the top branch really intended AB => AB[+GAB], but made an error, corrected in the 2nd commit. The problem is fundamentally ambiguous. We can argue which of the diff3 or anu/pijul heuristics are better. In practice I suspect the difference is not that large.
For completeness, here's a guess of what diff3 does, assuming merging top into bottom, following https://blog.jcoglan.com/2017/05/08/merging-with-diff3.
[bottom] => A [+X] B
[top] => AB [+GAB]
B O T
A A A
+X
B B B
+G
+A
+B