I've just been made aware of what I apparently did wrong: Scumtron was not listed as a co-author on the either of our NG2 submissions. I believe I understand where things went wrong here.
This is Xipo's first PM to me, after I announced I was working on NG2. The last line is of importance, because I think this is the "promise" that I broke. I misunderstood this line: I assumed that Xipo had intended to reach out to Scumtron so that all three of us would actively work on the improved run. The run ended up being created by just Xipo and myself, so when I submitted the run, I did not include Scumtron as a co-author, since we did not get in contact with him and he had no actual input in the improvement. At no point did Xipo make me aware that omitting Scumtron was a broken promise, though. Across both submissions, for as long as they were up on the workbench, Xipo never got in contact with me and directly asked me to do this. He even provided me with an improvement for the second submission once it was already up, and he did not mention that I was doing anything wrong with the co-authorship.
This is a case of misunderstanding on my part, and I'd like that to be corrected no matter what, but I do need to stress how there was no communication from Xipo that anything was wrong in the first place, and if there was I would have swiftly fixed things.
EDIT: Scumtron has now been credited on both NG2 movies.