Of note, all of those 66 planes are leased from and operated by other cargo airlines, like Atlas Air. Amazon doesn't own the planes or employ the pilots, they just lease the ability to paint the Amazon logo on the side and have the planes fly routes they choose.
It's unclear if the planes mentioned in the OP are talking about Amazon actually outright purchasing planes (which would be a first) or if they are just leasing more.
edit: Actually the article does say this at the bottom:
>Amazon launched its own air cargo fleet in 2016 and, prior to Tuesday's news, the company leased 80 planes, but the move is the first time the company has bought their own