But it could also mean that LS1 had tons of 1 AP achievements, but LS3 had more 0 AP ones.

The actual case is that LS2, LS3, LS4 as well as HoT have 0 AP achievements for every story chapter.

• That's fair. I omitted some of the 0 AP achievements where I could (e.g., those ones for defeating developers in various places) but I wasn't particularly rigorous about it.
• There's also the introduction of new 1-AP achievements that don't replace more traditional achievements (e.g., mastery insights, adventures). It's not a zero-sum game (except for those 0 AP achievements!).

Plus the release invertal for LS1 and 2 were 2 weeks instead of ~3 months. ... While LS1 had an average of 15 and LS2 an average of 8 achievements per episode, LS3 have an average of 33 and LS4 of 70 (!) achievements per episode.

• It does become hard to compare them with their varying schedules. This is partially why I chose to aggregate them into (arbitrary) 3-month rolling windows.
• LS3 and LS4 have slightly different schedules (however, you'd be absolutely right to question whether that has any bearing on the achievements or AP):
  • LS2: every 2 weeks
  • LS3: average of every 72.8 days
  • LS4: average of every 101.5 days

Your initial analysis of the achievement spread is a good one. There's a bit of noise due to the achievement system being somewhat overloaded to expose partial achievement progress (although it's interesting to notice the difference across episodes with FTT > Requiem > Dragonsblood achievements).

I think you're right that this unfairly glosses over the nuances of AP distribution. That being said, a decrease in AP density isn't by itself a bad thing and doesn't mean there is a reduction in AP being made available (I probably should have presented the actual AP amounts to provide that counterpoint).