One other difference. Supernetting was a technique specific to class C networks. It was a way to concatenate class C networks to form a larger network as Lazaros highlighted which was connected to a single router interface (in some sense it looked like a one armed router). So, technically its not a classless scheme. The default mask that you start with is /24 used for class C addresses. You need to borrow bits form the third octet starting from least signicant bit moving towards the most significant bit (opposite of subnetting) to define your supernet mask.
CIDR on the other hand does not recognize classes. So with CIDR in place, superneetting is no longer required.
Supernetting was specified in RFC 1338 which was published in June 1992 whereas CIDR was first specified in RFC 1519 which was published much later in September 1993. These are two independant techniques and CIDR obsoletes Supernetting.
1 Like