Technically, it doesn't matter what number you stick after the first stop in a /8 network. Might as well write it as 10.X.X.X/8, it's all the same. I'm not sure where I've seen this done, but >90% certain some of the ip commands output this kind of CIDRs by simply concatenating the IP of the node you are querying and the size of the network.
You encounter that all the time in the networking world - formally the bits outside of the prefix length should be 0, but informally you'll often see set bits. Often it is a way to say "the subnet that includes this ip address". So 192.254.33.12/16 or something. Could just be a typo, too. Regardless, it doesn't really matter what is in the bits outside of the prefix length, because they get zeroed out when used.
Huh? Do they mean 10.0.0.0/8, or 10.2.0.0/16 (/15 would also work), or is this a new CIDR notation? Something specific to AWS, maybe?