The reporter rightly queried other researchers about this article, and all of them were skeptical that a "supply shock" could be the cause, or even the main cause. My own skepticism is because the death rate went down many months before any sign of shortage appeared.

I haven't read the paywalled Science paper, but The Economist extracted a graph which shows that the purity of Fentanyl pills was stable till the first months of 2024, then dropped sharply. The purity of the powder peaked in 2023, then went down in 2024, back to its older levels. They suppose that it proves the supply was short, but another researcher even states that the supply of Fentanyl precursors didn't change until the end of 2024.

Anyway, the epidemic plateaued by the start of 2022, then went down after August 2023; Source https://www.cdc.gov/nchs/nvss/vsrr/drug-overdose-data.htm

Why did the death rate slow down for one year, then go down many months before any sign of supply changes?

The article says that deaths peaked in mid 2023. Narcan was approved for over the counter use in March 2023.

That suggests a plausible alternative cause.