They have different focal lengths, so even if the glass was originally for a KH-11 it couldn't have been used in Hubble "as is", which makes it nothing more than a rough polished mirror blank.

Why would it need to be used in Hubble? We don't even know if these satellites are KH-11s. They might be some other design that is capable of fulfilling a NASA mission. The point is that Hubble is suspected to be based on a spy satellite so there is potential technology transfer. Maybe these are just useful as parts.