That seems largely correct, Monero came out in 2014, so I think it was a combination of law enforcement becoming more familiar with how to track BTC payments and markets realizing there's a better alternative.
I think the fact that the U.S. Government put out a bounty for cracking Monero shows that it's working fairly well so far
The thing about monero, is that even if it is impossible to track today, all the transactions are still in the public blockchain, even if heavily obfuscated. It is quite possible that it eventually will be cracked and all historical transactions deobfuscated. Then it becomes as simple to track things down as bitcoin is today.
If this ever happens, it could lead to a massive wave of crime resolution on par with what happened when DNA testing became cheap and ubiquitous.
Because of this, when it comes to significant organized crime, physical cash and seedy banks like Chase and Deustche Bank are still king.
Bitcoin is for people who don't mind living in the light.
The article mentions several times how criminal naivete over the untraceability of Bitcoin aided the investigation - so why let the cat out of the bag now? It might be just that the IRS thinks it is not getting enough respect, but another theory is that Bitcoin is becoming known as being traceable, alternatives like Monero are currently effective, and the hope is to spread FUD by association and the implicit threat of future retroactive traceability.
To be clear, if that is the case, I have no objection.
I think Lex Friedman did interview with a drug dealer and he told this as well.
At the same time I don't use Monero for example as I'm not a drug dealer and they are using really complex cryptography for me to verify and trust.