HT12e: How is this a 2¹² encoder?
You're conflating a couple of definitions of the word "encoder". In this case, the chip is taking 12 binary inputs and encoding them as a serial word along with some synchronization information. The resulting word has 212 different states or values.

There is a different kind of (combinatorial) circuit that accepts 2N separate inputs and encodes the highest-valued one as an N-bit binary number. This is known as a "priority encoder".