I worked at Google during that period. It's not quite as simple as "Python > C++" although yes, for sure, you're going to produce new UI features at a far faster rate working in a high level language like Python than working with C++. Absolutely that was a factor. I don't think Google Video had hundreds of developers vs 20 though. That doesn't sound right to me. The real reasons YouTube won were strategic, not purely…
Python has some of the capabilities, but surely those libraries are also written in C/C++