I think the best take away is that they were both awful about open communication. Instead of sitting down and talking to each other (as each of them were at least somewhat inclined to do and felt they should have), they procrastinated and wallowed in self-pity and teen angst.
Even the flash forward to Lemon's attempt to push Sunao further away is the epitome of just not being open and honest about what she wants. It's why we've had the whole story to read so far.
Even as a fairly stalwart defender of this manga, I don't know that I buy everything from the flashback POVs, but I do think they ultimately accomplish what they needed to and I do buy most of Lemon's motivations. I was once a moody, depressed, self-destructive (emphasis on self-destructive) teenager once. I still carry some of that baggage over a decade later so I can't say it's all that unrealistic.
Also, it's not filler. Filler is frilly nonsense that doesn't flesh out the story. This is the exact opposite of filler.
Zero story advancement with many chapters of zero-progress retcon flashbacks handled like this is still filler, or padding, or time-wasting chapters meant to prolong a series.
We got:
1. Mean girl plotline introduced
and resolved before the filler flashback is over, leaving Lemon with the same romantic feelings she had at flashback start and end, and
2. Sunao being unchanged from how he is now except a single page where Sunao decides "I guess I'll just think of her as the witch" to try and hold the original stories new holes together with dental floss.
This flashback degrades the quality of the older chapters on a re-read. And nothing of value is gained from it. It just filled space to buy time for the actual story to catch up.
That's the reason all the people defending it keep adding qualifiers such as
"I don't know I buy everything"
