Originally, and perhaps at the time this "backdoor" was created, Skype wasn't Microsoft.

Skype was around for along time before Microsoft bought it and changed its architecture and design.

This. It could well have been a backdoor that predates the "superpeer" change MS introduced right after acquisition. Skype was already under pressure from European authorities at the time, to provide intercept capabilities; European criminal networks (mafia etc) were early adopters and everybody knew it.

That collaboration looks like the same mechanism used to adhere to warrants, subpoenas, and NSLs. Do you think that any internet service in the world doesn't have similar mechanisms to comply with law enforcement requests in their home country?

I think you are misleadingly using the word "collaboration".

I also think you have failed to understand the article correctly; there is no reference to client side collection. Take another look.

Agreed. I'm just suggesting if someone were to ask for a client backdoor they could have one, just like they could ask for a server feed.