It is probably fine if he is giving a portion to the people he interviewed. People go to mixergy to hear from them. It does also push people to check out interviews before they go behind the pay wall.
However, I think this is a really bad business move, second only to using a fake name. It creates distrust and devalues the community aspect of the product.
A really bad business move is doing something that isn't sustainable. Given that giving his interviews away for free isn't sustainable, putting his content behind a paywall is probably an experiment to see if he can build a sustainable business.
However, I think this is a really bad business move, second only to using a fake name. It creates distrust and devalues the community aspect of the product.