What makes you say "they did more complicated decryption tasks"?
This speaks to a larger question of "Why do government agencies ask for help in decryption?"
The most common two reasons are chain of custody/saving face and they actually can't do it. Government agencies are good generally, but they're not superpowers.
Look at Apple vs FBI for my first reason: They could ask the NSA, but they haven't. It'd be a breach of the NSA's actual directive but it's not like they haven't surpassed their directive in the future. It'd draw public scrutiny and may encounter issues in the court if they cite it as evidence.
If a major company develops top tier security, they can't just constantly pull a private key to decrypt it or a major 0day exploit out of their a**. WhatsApp may have more information regarding their private keys and the like assuming they're even all signed with a private key WhatsApp is in possession of, or they sincerely may have taken themselves out of the equation as a trusted authority and made the process autonomous, in which case WhatsApp cannot help.
What Alexey is referring to is the fact that this would be more common if it weren't for the fact that so many companies just fold to government pressure and hand everything over.