IPv6 – Why is fe80::/10 Reserved for Link Local Addresses?

Most IPv6 routing protocols use the link-local address of the next hop. Remember that the only purpose of the next hop address is to determine the MAC address of that next hop (on ethernet at least. Other media use similar mechanisms). Except for this lookup the address itself is never used. So it doesn't matter that you only see the link-local address. It is enough for routing purposes.

And that also means that if you want you can build your whole core network using only link-locals and loopbacks. Mught be a bit harder to debug though :)

The document to which you link claims a specific IOS version. It may be incorrect; that would not be unusual in Cisco documentation.

In reality, having tested this on several IOS 15.X versions, you do not specify a prefix length on a link-local address since it is implicit for the link local address, as you saw in your CLI help (ipv6 address ? shows X:X:X:X::X IPv6 link-local address, the address without a prefix length). You do specify the prefix length on a non-link-local IPv6 address since there is no implicit prefix length.

Personally, I think specifying specific link-local addresses on your interfaces is going to be a giant pain, with little to no gain. Just enabling IPv6 on an interface will assign a unique link-local address, and that is all you really need. There is really no security risk about having the Interface ID being derived from the MAC address, since the address will never be seen off-link, and it won't be tracked across the Internet as a Global Scope address could be.