The article addresses this. They have experienced developers on-hand to assist and ensure standards are maintained. Getting the time to learn is a management problem, not a developer problem.
I've taught C# and the .Net framework to several teams over the last two years. The good developers pick it up without much problem. But, we usually start with several sprints of co-located group programming (for complete teams learning the stack).
Agreed, it's important to that the entire teams understands when someone is learning a new skill set or language. We try to allocate more time in our sprint for not only the developer who is learning but also for another developer on the team to help ramp them up.
It takes a bit of working with the PM to determine what is a good task that isn't super time sensitive but it's typically a solvable problem.
I've taught C# and the .Net framework to several teams over the last two years. The good developers pick it up without much problem. But, we usually start with several sprints of co-located group programming (for complete teams learning the stack).