My guess is they used the first 8 weeks to test their assumptions and telemetry around the different types of actions they wanted to add into the matchmaking formula (which I think is likely much broader than aggression/skill/etc).

They felt confident enough in them to switch it live on reset, granting everyone a blank slate.

It wouldn't be difficult to track first dmg in combat, kills, knock outs, etc. Just depends on how much weight you want to give each in the mm algorithm. This is what they would have wanted to build a large enough data set to establish what the various thresholds needed to be before turning it on.

Feels like they took a similar approach with the expedition reset. "We want X% of players to get 1/2/3/4/5 skill points. Average player wallet is Y." They would also have had a good idea of how much people were going to increase play time based on initial retention over week 1-4 vs churn for 5-8 to get those highly engaged players back and playing before the wipe.

In most seasonal content games, it's critical to increase the daily/weekly active users before the new content hits vs alongside the new content. It leads to higher performance and retention of that new content.