Good eye! That was a typo. It should read as "colon notation". It is just a visual representation of 128 bits (16 bytes) with each byte separated by colon ":". I have updated the readme with an example. One place where you will see this notation visually is MAC addresses.
UUID RFC use <timestamp, sequence, node id> layout which is different from the layout of snowid scheme which is <timestamp, node id, sequence>. Also UUID RFC timestamp can only use up-to 60 bits whereas snowid has the full 64 bits to spare. They server different use-cases.
